First, let's clear up a misconception.
Armor Penetration gives you a constant %damage bonus, regardless of how much armor the enemy has see here.
It doesn't matter if your enemy has 300 armor or 15 -- penetrating 10 of that will have the same effect on total DPS increase.
That said, let's take a look at why Armor Penetration is preferred, even with its linearity.
Attack Speed is easy to acquire via items.
Armor Penetration is not.
You can more than match the effects of attack speed runes with a single dagger - 420 gold.
The cheapest item with Armor Penetration is the Brutalizer at 1337 total gold. By this metric alone, Armor penetration runes are worth approximately three times that of attack speed runes.
It is easier to use items to stack Attack Speed over Armor Penetration -- but to get the highest DPS, you want to stack both.
Given that the amount of Armor Penetration available via items is strictly limited, if one's goal is to achieve maximal values for both Armor Penetration and Attack Speed, runes for Armor Penetration are much better than that of Attack Speed.
Since the big remake in Season 3 on Black Cleaver, and subsequent change to Last Whisper, most of the answers on this page are no longer accurate.
Black Cleaver: The Nami patch changed several things, among these were the way penetration works as well as the stats on the Black Cleaver itself. Now, the Black Cleaver provides health, attack damage, cooldown reduction, and its distinctive armor penetration. These changes make the Black Cleaver a far more appealing option to AD bruisers (such as Darius, Garen, etc.). The removal of its attack speed stat makes it less attractive to AD carries, although a little bit of health never hurts.
Last Whisper: Last Whisper, on the hand, has remained a key item in an AD carry's arsenal. Interestingly enough, Last Whisper's passive is no longer armor penetration, per say, but causes all physical damage to ignore 35% of the target's armor (applied before any penetration). While this doesn't seem all that different from normal armor penetration, the key is in the parenthesis: applied before any penetration. This means that Last Whisper's passive has a key difference in the way that it affects armor than normal armor penetration.
You see, percent armor penetration stacks multiplicatively. For example, assume that you have two items, one that has 15% armor penetration and one that has 25% armor penetration. Your total armor penetration is calculated like so:
- First, change the armor penetration into reduced armor percentage (15% armor penetration = 85% armor reduction and 25% armor penetration = 75% reduced armor)
- Multiply all your sources of percent armor penetration (85% * 75% = ~64% reduced armor)
- Change the armor reduction back and you have 36% armor penetration.
- This new penetration stat is now applied to your target's armor.
Now, Last Whisper is applied before any armor penetration, so it doesn't follow the above process. Let's see how this affects your enemies.
Set 1: 20 armor penetration and 25% armor penetration (Youmuu's Ghostblade + Black Cleaver, a common armor pen build for bruisers)
Set 2: 10% armor penetration and Last Whisper (10% armor pen from marks, which are more common on AD carries than bruisers)
Target 1 has 100 armor (assumed armor for non-tanks). Set 1 would reduce it to 75 armor and then subtract the flat penetration for 55 armor. Set 2 would apply Last Whisper first to reduce the target to 65 armor, followed by the percent armor penetration to bring it to ~59 armor. Hmm... it seems Set 1 wins this round.
Target 2 has 250 armor (assumed armor for tanks). Set 1 would reduce that to ~188 armor and subtract from flat armor penetration for ~160 armor. Set 2, on the hand, applies Last Whisper first to bring it to ~162 armor, followed by the percent armor penetration to ~146 armor. Clearly, Set 2 is far more useful for killing tanks.
And this is why Last Whisper should be chosen on AD carries rather than Black Cleaver. The job of the AD carry is to kill everything. Fast. As in, they should have been dead two weeks ago. Squishies are easily nuked by the others on your team, AP carry and bruiser, but only you have the damage and consistency to kill the tank.
Best Answer
Let's get some things clear: There is no cap on Armor penetration and there are not too many Items that provide it.
So let's start off with the runes and masteries. From runes you can get a total of 19.2 ArPen while from masteries you have 6% Armor Pen. Now if you pick darius (Champ with most armor penetration) You can start the game at level 1 with 19.2 and 11% ArPen.
If you continue your ArPen journey you will have to buy A Black Cleaver(10Flat), A Yomuus Ghostblade(20Flat), a Brutalizer(10Flat) and a Last Whisper(35%). With darius and his E maxed (25%) + all the runes this gives you a total of 69.2 Flat and 48.6% since percentage armor penetration stacks multiplicatively like Soulcreeper has pointed out in his answer.
That's a pretty huge amount but I wouldn't consider it viable because it's purely glasscannon and has no real use to the team. It's usually better to either go for some crit or survivability depending on the champion you're playing.