The top 3 ad ranged champs according to a lot of the community are Vayne, Graves, and Caitlyn. Arguably, Tristana, Kog, and Sivir fill the next 3 spots, and Ashe, Corki, Miss Fortune and Twitch take up the last 4. I don't count Ezreal, I never really see him played in that ad slot. (Note, that my top picks listed may or may not reflect popular opinion.)
You should familiarize yourself with as many ad champions as you can, the more you play, the better off you're going to be in countering your enemy ad ranged. Also, by playing all of the ranged ad, you will know their strengths and weaknesses, which is crucial to winning your lane.
Also, play some supports. Knowing your lane partners champion, and knowing what the enemy support can and cannot do is also good to know.