Unlikely to be the highest possible, but the highest typical 20th-level ability score, assuming you’re focusing on it, is 36:

• 18 base
• +2 racial
• +5 from levels
• +6 from enhancement bonuses
• +5 from wish or a manual

For mental scores, there is also the +3 bonus you can receive for being venerable. These usually come with a −6 penalty to all physical scores, so this is not generally considered optimal unless you have some way of ignoring the penalties (losing 60 HP and taking a −3 to initiative isn’t worth +1 to your DC, even if you have no other uses for physical scores). But you could get to 39 by being venerable.

Anything beyond this requires special bonuses that are not typically available: unusual items that grant non-enhancement bonuses to an ability score, feats or class features that improve an ability score, and so on. Strength and Constitution get a lot of these, and Dexterity some, but mental ability scores only receive such bonuses rarely. I am not aware of any for Charisma, though I don’t doubt there are some.

OK, so we can do this by leveraging five prestige classes:¹

• Arcane hierophant—dual advance arcane and divine spellcasting, 10/10 levels (Races of the Wild)
• Blighter—9th-level divine spells in 9 effective class levels (Complete Divine)
• Fochlucan lyrist—dual advance arcane and divine spellcasting, 10/10 levels (Complete Adventurer)
• Sublime chord—9th-level arcane spells in 9 effective class levels (Complete Arcane)
• Ur-priest—9th-level divine spells in 9 effective class levels (Complete Divine)

Between these, we have 40 levels’ worth of effective spellcasting levels, and we have three classes that can reach three different sets of 9ths with a total of 27 effective spellcasting levels. If we can combine these effectively, we can manage enough effective spellcasting levels to gain 9ths three times.

Relevant abuses, in ascending order of my personal tolerance for them in a real game:

• Using Alternate Source Spell to advance a divine spellcasting class with a prestige class that is trying to advance an arcane spellcasting class. (I don’t really personally think this works, and have come up with another answer that avoids it, though there are definitely trade-offs.)

• Using a friendly 9th-level bard’s inspire greatness during our 3rd or 4th level-up to meet skill rank requirements earlier than should be possible. (This works, strictly speaking, but I would basically never allow it in my games.)

• Using Versatile Spellcaster to qualify as having spells a level higher than we otherwise could cast. (It works, but probably wouldn’t ever allow it.)

• Repeatedly ignoring prerequisites after taking the first level in a prestige class. (This definitely works, and I might allow it in my games depending on circumstance.)

• Qualifying for a prestige class using an item. (This definitely works and I allow it routinely in my games; you should too.)

More details on how these work and what they accomplish for us below.

1. _{This is not the only way to do it, just a way to do it. Note here that the existence of these classes makes a trivial answer like Pun-pun possible. Pun-pun could choose to take ur-priest at 2nd, blighter at 3rd, and sublime-chord at 4th, and then just use 8 levels in mystic theurge to get 9ths in sublime chord and one of the divine classes, and then take the remaining 8 levels in the remaining divine class to get 9ths in that too. This is pointless, though, since he already had all the 9th-level spells in the game at 1st.}

But these prestige classes have a lot of problems with them:

These are all prestige classes

That means levels wasted prior to being able to enter, plus we have to actually meet all the prerequisites. If we don’t get into any of them in 1st to 5th, then we have 15 levels remaining—3 to the first level in each class, and then 12 levels of dual-advancement. That works out to 24 effective levels’ worth of dual advancement, as well as 3 for the original level in each class—just barely meeting our requirement of 27. So every single level after 5th has to be either the first level in one of blighter, sublime chord, or ur-priest, or else it has to be advancing our level in two of those classes.

Sublime chord has brutal requirements and cannot be entered before 11th

Nothing much to do about this one: gaining 13 ranks in two different skills before 10th level is hard. We could push it back to 8th if we really needed to, but it’s probably better to just accept this and take sublime chord at 11th. That means that 8 of our remaining 18 effective spellcasting levels gained across 12th to 20th are going to have to be dedicated to sublime chord, so our other classes are going to have to be ready to achieve 9ths with the remaining 10.

We don’t have a class that advances two divine spellcasting classes

We already admitted defeat on sublime chord, and are leaving it until 11th. But our dual-advancement classes are all arcane/divine. In fact, there is no divine/divine dual-advancement class (there is an arcane/arcane one in the ultimate magus, Complete Mage). How do we deal with that?

The answer is, arguably, Alternate Source Spell from Dragon vol. 325. Since we are taking both bard and druid levels, we don’t have to cheese qualifying for that feat, and it means our 3rd-level druid spells can be arcane. Is that good enough for arcane hierophant and Fochlucan lyrist? No one knows! But we’re going to assume it is.

Blighter requires 3rd-level spells

This is problematic simply because we need a level of bard (to get the bardic music we’ll eventually need for sublime chord), so we can’t simply start as a 5th-level druid. So we’ll have to cheese at least one extra spell level out of a 3rd-level druid, minimum.

Since we have a level of bard anyway, the easiest way to do that is Versatile Spellcaster from Races of the Dragon, which we unambiguously qualify for and unambiguously can use on our druid spells despite that feat’s requirements. Bam, our 1st-level bard/3rd-level druid can cast 3rd-level druid spells.

Ur-priest and other divine spellcasting

Ur-priest has a requirement that

Special: The character must have no ability to cast divine spells. If such spellcasting ability was previously possessed (as with an ex-cleric), that ability is forever forsaken.

That means taking one’s first level of ur-priest nukes any divine spellcasting you had. However, it does not mean that you can never have other divine spellcasting ever again—the forsaking is explicitly about spellcasting ability “previously possessed.” The first line, on the other hand, can be safely violated once we have taken our first level of ur-priest: we won’t lose our ur-priest class features, or even our right to take more ur-priest levels, per the Dungeon Master’s Guide.

So we have to take blighter after our first level of ur-priest, or else ur-priest is going to destroy our blighter spellcasting.

Arcane hierophant and blighter both require BAB +4

Unfortunately, once we take a level in each of bard and druid, which are mandatory for sublime chord and blighter, respectively, it becomes impossible to get BAB +4 before 7th level, when we want to take blighter, unless we use fractional BAB (which everyone should, but nonetheless).

And simply delaying blighter and/or arcane hierophant another level doesn’t work out to get us 9ths.

We manage to hit BAB +4 if we can take a full-BAB class instead of bard at 1st. Paladin could actually fit the bill, since the harmonic knight variant from Champion of Valor gets 1/day inspire courage +1 instead of at-will detect evil, but only if we waive alignment and religion requirements as harmonic knight requires Milil as patron and that won’t jive with ur-priest at all.

Instead, we could use Augustinius’ Folly, a “slightly-cursed” suit of +2 banded mail from Dragon vol. 324, to get “inspire courage once per day as an 8th-level bard,” and then take any other full-BAB class at 1st. Duskblade is probably best just because its an arcane spellcaster, easing our access to Alternate Source Spell and counting towards ur-priest CL.

But for games without Dragon and without waiving alignment and religion requirements, this one we’d have to just bite the bullet on: even if we cheese our spells harder, 1st-level bard/1st-level druid is a minimum for us and once we have that, we are not going to get BAB +4 in five levels without fractional BAB.

Since the question does allow us to do any of these things, however, I am going to favor the bard level. Skill points, why not? Plus Augustinius’ Folly is slightly-cursed, and paladins come with baggage.

^{(thanks to @Forrestfire for pointing out Augustinius’ Folly.)}

Blighter requires us to be an ex-druid, but arcane hierophant requires a druid class feature

Blighters must be ex-druids, while arcane hierophant requires trackless step, a druid class feature. And obnoxiously, ex-druids don’t just lose spellcasting and supernatural abilities, but instead lose all druid class features beyond the base chassis and armor and weapon proficiencies.

An arguable solution here is to get trackless step elsewhere. There aren’t many options, but bamboo spirit folk from Oriental Adventures gain it as a racial feature. That solves that—arguably. Arcane hierophant does specify “trackless step class feature,” and bamboo spirit folk is a race, not a class. Since we’re being pedantic on a number of other points to our advantage, choosing to not be pedantic here is problematic.

So let’s embrace pedantry. A 9th-level bard can perform inspire greatness, making us count as having 2 HD more than we do. If we level up—say, to 4th—under this effect, we count as being two levels higher—6th—and thus our skill rank maximum increases by 2 as well—to 9. That gives us room to put 8 ranks in Knowledge (religion) and Spellcraft at 4th, so when we level up to 5th, we can take ur-priest. We can ease the timing requirements here by having it be an undead or warforged bard, as they can play inspire greatness indefinitely without tiring or otherwise needing to stop, guaranteeing that our level-up happens during the effect. So make friends with (or hire) any mid-level undead or warforged bards you meet. Alternatively, a regular bard, and a friendly 7th-level psion to cast psychic reformation to re-allocate your skill points instantaneously while the bard plays.

Starting ur-priest early buys us an extra level: since we are ahead of schedule on ur-priest, we only need 26 effective levels in the 6th-20th range. We still take ur-priest at 6th as planned (but now the second level in that class), and we reach BAB +4 to qualify for blighter (assuming either fractional BAB or harmonic knight), again just like we planned it. But this is where that extra level we bought ourselves comes in: we can delay blighter one level, and BAB +4 also qualifies us for arcane hierophant—and we haven’t taken blighter yet, so we still have trackless step from our druid levels. Now that we’ve taken the 1st level of arcane hierophant, again per Dungeon Master’s Guide, we can continue taking levels in it even after becoming an ex-druid and blighter.

Note that the first level of arcane hierophant, in this version, is advancing ur-priest and bard. Having 2nd-level bard spellcasting isn’t exactly thrilling, but just pointing it out: we can’t advance blighter, since we haven’t taken our first level in it yet, and we can’t advance druid because...

We lose our druid spellcasting when we take ur-priest

Yes, we do. It’s gone. We no longer can cast 3rd-level spells—but we are an ex-druid who was capable of doing so, meeting blighter’s requirement. And since we can use Versatile Spell on both ur-priest and blighter, they both have 2nd-level spells. Alternate Spell Source makes one of them arcane, and bam, we qualify for arcane hierophant.

What about feats?

Assuming we go the early-ur-priest route, we need:

• Iron Will and Spell Focus (evil) before 5th in order to qualify for ur-priest.

• Versatile Spell before we lose druid spellcasting (at 5th) in order to qualify for blighter.

• Alternate Source Spell before 6th in order to qualify for arcane hierophant.

We only get feats at 1st and 3rd—two short. That means we need some source of bonus feats. Flaws are the easiest, if that variant is in play, and if not we could always be human for one and abuse the Otyugh Hole from Complete Scoundrel to get Iron Will for the other.

If we go the bamboo spirit folk route, we lose the human bonus feat, but we also don’t need Alternate Source Spell until 8th, so we can pick it up at 6th. The level taken at 5th under this approach is also completely free to do whatever, potentially allowing us to pick up one of the others as a bonus feat.

Fochlucan lyrist requires evasion, and also a boatload of skills

Evasion should just be gotten with a ring of evasion. Feel free to sell it after you take the first level of lyrist.

The skills, on the other hand, may be simply not worth it. The requirements are considerable, almost all of your levels are in 2+Int or 4+Int classes—and with your needs for Wisdom and Charisma, I wouldn’t expect staggering Intelligence—and perhaps most importantly, sublime chord requires a ton of skills too without a whole lot of overlap.

So maybe just ditch Fochlucan lyrist, amusing as it is, and just use arcane hierophant until 18th (when you finish it) and take the last two levels in mystic theurge.

Final Build

A neutral evil human with the following levels has the indicated level of spells in each of the listed classes:

1. _{We lose druid spellcasting at 5th when we take our first level of ur-priest, and we lose all other druid class features at 8th when we take our first level of blighter.}

2. _{In order to meet the skill rank requirements for ur-priest before 5th, we must have taken our 3rd or 4th level while under the effects of inspire greatness. An undead or warforged bard makes the timing of this much easier.}

3. _{In order to meet the BAB +4 requirement, the campaign must be using fractional BAB. Without that, we could use a full-BAB class instead of bard at 1st and then gain inspire courage from Augustinius’ Folly, or else a houserule waiving alignment and religion requirements could allow us to substitute harmonic knight paladin for bard and reach BAB +4 without fractionals.}

4. _{The exact breakdown of arcane hierophant and Fochlucan lyrist levels is really up to you. But since you don’t have a familiar (unless you take Obtain Familiar, which you could), and may not have an animal companion either (depending on how the DM interprets an ex-druid arcane hierophant), I figure the music progression of Fochlucan lyrist is superior. On the other hand, the skills required for Fochlucan lyrist may well be prohibitive and you may prefer to skip it altogether, finishing arcane hierophant at 18th and then taking two levels of mystic theurge.}

I quite like how this build, despite being made for the cheesiest of reasons, actually looks playable as a real character. Levels 5th to 8th or so are going to be a bit painful, but not unplayable. And the progression suggests an interesting character arc: a bard/druid who forsakes both gods and nature and becomes a stark nihilist, but rediscovers the magic of music later in his career and weaves that into the anathemic magics he commands.