Not Updated For Current Season
There are many different runes options that a person has for when playing an ad champion, however, for basic and most optimal results on most ad champions, it is best if gone with:
Generally, there are actually two options you can go with, these being ARMOR PENETRATION or FLAT ATTACK DAMAGE
Armor Penetration- Armor penetration is generally more often used because of the it allows your basic attacks to penetrate the enemy's armor. It's great early on because of the fact that it gives a person great damage output to enemies who start out with higher armor. Think of it like this, If an enemy has 100 armor and you have 10 armor penetration then when you attack the damage is calculated as such: 100-90 and then your attack damage. Your attacks will go through 10 of the enemies armor and then the actual calculation from armor to attack damage ratio occurs.
Flat Attack Damage- I have found that going with flat attack damage reds are the better option because of the fact that it helps you earlier with last hitting minions and harassing. Armor penetration does work on minions, but attack damage gives the summoner more damage on the minions because i have found that one always thinks they have enough damage to kill a minion and then when they attack, it wasn't enough. Part 2 is because of the fact that most carries have a spell that takes damage from their bonus or basic attack damage. Unless it accounts for on-hit effects, then armor penetration doesn't apply to this and makes the spells do less damage (examples are Ashe's Volley or Caitlyn's Piltover Peacemaker).
Greater Seal of Resilience- For almost all Ranged AD Carries, this seal is used. Because Ranged AD Carries usually go bottom lane, they will be squaring off against other attack damage dealers. This small boost of 13 armor will help out and allow the ad carry to survive longer because they take less damage than they usually would from their opponent's attacks, allowing them to sustain and stay in lane longer to farm up and get the needed items on their first trip back to the base.