For early Game your best bet would be:
Red: Armor Pen
Yellow: Flat Armor
Blue: (Your choice, I normally base this one depending on who I am against, Magic Resist, Attack Speed, Attack Damage, Cooldown Reduction or Mana Regen are all good choices.)
Quints: Attack Damage or Armor Pen (I prefer damage, I find you get enough armor pen with your masteries and reds to be able to use quints as extra damage source, if you are having trouble with armor or they are stacking armor just buy an armor pen item in late game.)
That will give you a good rounded opening with the ability to be very aggressive (because of your armor) if you find your opponent leaving themselves open or if they over extend.
Magic resistance stacks additively, and is calculated via this formula (assuming positive MR):
Damage taken = Raw damage * (100 / 100 + MR)
25 magic resistance → ×0.8 incoming magic damage (20% reduction).
100 magic resistance → ×0.5 incoming magic damage (50% reduction).
source
Magic penetration is applied in this order:
- Flat magic resistance reduction
- Percentage magic resistance reduction
- Flat magic penetration
- Percentage magic penetration
source
Magic penetration stacks multiplicatively. Void staff (40% magic pen) and the Arcane Knowledge mastery (10% magic pen) together equal 46% magic pen. source
We can see that the types of magic pen are very important, because buying flat magic pen (sorcerer's shoes) makes % magic pen (void staff) less effective. Unless the enemy has a lot of MR (ex: Galio), it's not worth it to buy multiple types of magic pen.
Compare some examples with a base MR of 30. With 0 magic pen, 100 magic damage does ~77 damage. With Arcane Knowledge, we ignore 10% of 30 MR (27 MR total) and deal ~79 damage. With sorcerer's shoes, we ignore 20 MR (10 MR total) for ~91 damage. With shoes and arcane knowledge, we ignore 20 MR + 10% of 10 MR (9 MR total), or ~92 damage.
We can see that for low values of MR, increasing Magic Pen provides very little gain. It also shows how effective flat MR is vs low amounts of MR, whereas percent reduction is not.
Now let's consider a large MR, say 200 and 500 magic damage. With 0 magic pen, that's ~167 magic damage. Arcane Knowledge reduces the MR to 180, or ~179 damage. Sorcerer shoe's are the exact same here. With shoes and arcane knowledge, we ignore 20 MR + 10% of 180 MR (162 MR total), or ~190 damage. If we add a void staff to these 2 (discounting it's bonus AP for simplicity), we get 200-20-72-11 (97 MR total), or ~254 damage.
It's safe to say at this point that unless the champion is stacking MR, you'll do more damage buying AP instead of Magic pen. A notable exception to this rule is Soraka, because her passive provides a substantial MR aura.
tl;dr: Runes, masteries, and sorcerer's shoes provide enough magic pen vs most champions. Only buy additional magic pen if the enemy champion has over 100 MR.
Best Answer
Depend agains who you're going and what champion you pick.
MR runes are very important to survival in the lane (keep in mind that you want to stay as long as possible in the lane and farming/leveling), and usually you're going against another Magic User (ap). Glyphs are the way to go in those runes
Armor runes are also important in the lane for survival, the minions may kill you if you keep receiving damage (usually when you go after your opponent and go behind the enemy minions, the minions will turn up on you). These runes also help a little in late game. Seals are amazing for those runes.
Armor Pen. runes are not so much of use, if you are going as Fizz. They are great if you are Vayne for example. But if you want those (as Vayne for example) go for Marks and/or Quintessence
Magic Pen. runes are great for you, in early, mid and end game. Marks are usually enough, but you can get some Quintessence too.
Hybrid Penetration. runes are excellent on Fizz in the early game, but in the end game, you main damage will be upon the magic damage, since you'll hardly be able to do 3 hits in a row against a target (usually, you'll just jump, pop the ultimate, then scape from the mess, while your team finish the opponents and the opponents try to kill you). In the early game, if you are aggressive, you'll be able to jump in behind the minions, hit once or twice each "dive" then retreat. The main point on the Armor pen is the damage on the minions.
It's always worth to test YOUR play-style, and pay attention to:
Based on that, you can pick your runes and also, if you want them to be Flat or Scalable.
There isn't a "best-rune-set" nor anything like that. But the set of runes that Valentin Grégoire said in his answer, is a good way: