World-of-warcraft – Holy priest professions for the most beneficial self applied stat boosts

Note that this has been edited to reflect the changes in Cataclysm / The Shattering, so the stat weighing is not final and a proper raid-balance of stats is not concrete.

Int provides you with spellpower and a larger mana pool, so it is naturally a strong stat.

Haste is also important, as it allows you to cast your spells faster, and to an extent, reduces the GCD (which makes a difference for instant cast spells). You get a good amount of haste passively through talents though (9% through Judgements of the Pure, 3% through Blazing Light, plus flat cast reductions on certain spells).

Crit is always nice to have, and it synnergizes nicely with some talents (Crit Holy Shocks reduce cast time on HL / DL, for example).

Mastery (at 80 and beyond) gives your heals a bonus shield equal to a % of the heal amount on your target. Shield effects have always been quite effective, and this more or less gives you a permanent Valan'yr effect to work with. I can see this being a very popular mastery, however we'll have to see how well it scales further on.

Spirit is now the go-to stat for mana regen as of The Shattering, and will continue to be so throughout Cataclysm. There may be a FEW items / enchants out there that provide MP5, but for the most part you're looking for spirit for your regen needs. It still remains to be seen just how much spirit is going to be needed to sustain throughout hardmode fights (especially with the ability to Judge Seal of Insight AND Divine Plea as decent mana regen abilities), but regardless, Spirit is the new regen stat.

As for leveling, just worry about Int, Crit, and to a lesser degree, Haste. Early on and leveling, a lot of healing will be done with Holy Shock and Word of Glory, which are both instant. Haste wont play a big role until later when you need to utilize your Flash of Light/Holy Light/Divine Light spells a bit more.

Update for Patch 4.0.6!

Leatherworking is now exactly as good as blacksmithing / enchanting / alchemy / inscription. Pick Jewelcrafting if you need that 1 extra stat, but the rest are totally a wash.

Original Answer

Jewelcrafting can give up to +81 to a primary Stat, whereas enchanting / blacksmithing / inscription etc. only give +80. (Exceedingly minor difference.)

Also worth looking into is Leatherworking, which offers unique arms-slot enchants that are unique in that they provide a primary stat bonus (such as strength) otherwise unobtainable as an enchantment in that slot.

In general, the gathering professions provide less useful benefits than the secondary professions, a possible exception being Mining, which provides health. (Strength is a better damage increaser than both crit rating from skinning or an activated haste+heal from herbalism).

So the bleeding edge optimization would indicate Leatherworking and Jewelcrafting as the best, in terms of pure +stats. Mining / Jewelcrafting is easier to level, and the bonus health from Mining is not wasted during the period before (if) you switch it for leatherworking.

Beginning with Wrath, and developed to completion in Cataclysm, class is largely meaningless when looking at profession bonuses. The core primary stats of Intellect / Spirit / Strength / Agility are usually all available within a single profession (if they're present at all), making class choice largely irrelevant.*

Even Tailoring isn't useless for melee characters, as it provides a +AP cloak enchant (though its proc-based benefit compared to simply passive +strength is debatable)