Originally Posted by Silent Reaper
Armor Penetration is far better.
Crit chance only gives you a small advantage early game. Attack speed is good, but I'd say Armor penetration is much better.
I'm a bit tired of hearing this. It is wrong, do some math, its not hard to figure out.
Which is better depends entirely on the enemy team and the hero you are playing as well as your planned item set.
Armor has diminishing returns, the more armor you have the less effective additional armor is, therefore armor penetration is the exact opposite. The less armor the opponent has and the more armor penetration you have, the more effective it is to get additional armor penetration. armor penetration is good early game, but scales poorly late game as enemy heroes get more armor. This can be fixed with an armor penetration item like last whisper. It also scales well the more you get, so last whisper with a black cleaver and armor penetration runes is an awesome build.
If you are up against high armor targets (rammus with 300 armor) then your armor penetration runes will be almost worthless, but if you are up against low armor targets it is great. If you don't plan on getting an armor penetration item (brutalizer, black cleaver or last whisper) then you probably should not get armor penetration runes.
Attack speed items are a percent of your attack speed, so heroes with higher natural attack speed get more benefit from attack speed runes.
Attack speed isn't that good on most heroes early game, as their natural attack speed is low. It scales well late game though, and its good on heroes with high natural speed (twitch, yi, tristanna etc . . .)
Critical damage runes give the most damage of any rune in the game, under the right circumstances. You need to build your hero to have a very high critical chance before critical damage runes will become effective. Critical damage is therefore bad early game because your critical chance is low, but it scales the best for late game damage if you build for high critical chance with items.
Critical chance runes give you a percentage increase in damage, and therefore if you have low natural damage they aren't very good. If you are not planning to get some damage items early then critical chance is not for you. High damage heroes (Sion with enrage on) and heroes that already have good damage items benefit greatly from critical chance. Also critical chance and critical damage work very well together, so if you get one, you should get both. It is a percent damage increase though so it is best on heroes with high damage. Get some damage items first, then go for critical chance. Critical chance can be good both early game and late, and it scales well late game getting better as you get more damage.
Critical chance and critical damage should almost always be done together. If you buy an infinity edge, then get a bloody phantom dancer as soon as possible or you are stupid. These two items just belong together, always.
Early game getting flat damage is nearly always better, late game go for damage multiplyers (attack speed, armor penetration, critical chance and critical damage are all ways of multiplying your damage) If you have over 300 damage and you are still buying flat damage items then you are doing it wrong.
Get the runes that will most benefit the hero and item build you are going for. If you plan on getting an Infinity edge then getting Critical chance runes is a great idea.