As some have noted, learning to become better ranged AD is different than which champ is best to use in real game play. Playing ashe, because of all her built in slows and her long range Ult, is very different than playing all other ranged ADs. I disagree that she makes you a better generalist ranged AD. her game play too specific.

despite her getting the least votes, MF is one of the best to learn with. Her lack of escape, the nature of her "skill" shot and most importantly the mechanics of her ult FORCES you to learn positioning and poking. The most important skill as an AD is correct positioning. 2nd most is knowing when to commit. If you can roll with MF, then you should be able to roll with most other ADCs.

I voted Vayne, if you take priority on her silver bolts and build her AS/Crit her burst is fantastic, and you can buy some cheaper items early game to really give you an edge and they build into far better items later in the game. The down side to playing Vayne is that you are going glass cannon with her unless the game lasts a long time, if you are going to be a viable AS/Crit Vayne, you aren't building any health or armor until your primary items are built.

CAITLYN. positioning with her is incredibly easy due to her range, as is poking the enemy AD carry during the laning phase. her abilities also suit a learning AD carry player because her q deals massive damage once you learn how to use it, her trap are amazing to line up a q or to escape , and her E ability allows her to escape any situation. her entire kit suit a new AD carry incredibly well.

In my opinion since I can't vote twice. Draven to force you to move and be aware of positioning. the second would actually be vayne. She teaches and will punish herself or opponent based on position. She also becomes fairly easy to last hit with.

i don't recommend trist or vayne as both of them are heavy damage and excel late game but are the two hardest ad carries to master. even in late game they are pretty squishy and can get easily focused and destroyed if you do not utilize proper positioning or mess up your abilities.

I voted Sivir because that's who I learned on. She's strong, that boomerang harass drives people insane, spell shield can save your life and she can do some great damage. Her Ult is awesome offensively and defensively(getting away).

Besides her I'd stay away from Vayne or Kog for learning, but once you get the idea of positioning down and how to do it pick them up. I suck at vayne and am still trying to learn Kog. They both just take a lot more skill than say, graves, who is awesome and I can just go in and burst people.

Ashe is the quintessential AD carry I feel like cuz it seems like almost everyone learned on her.

So I'm wondering this too, but I'm good at this role (its really easy). But I'm more wondering which is the best, In my opinion Graves, Ezreal, and Kog'Maw are all up there. Vayne is high up there if she can get a good start and I also forgot about Corki.

easy: ashe, caitlyn, sivir-straightforward playstyle, room for mistakes
medium: graves, miss-fortune, varus-great all game, more complicated playstyle
hard: corki, ezreal, tristana, vayne-reliant on teamwork, smart play and positioning to be effective
hardest: kog'maw-has only range and damages, little cc, immobile and no reliable escapes, teamwork and positioning is key

Ezreal isn't bad to pick up first at all. He has a comfy range, that is standard for AD carries. Not too long, not too short. It will let you practice your positioning without having to be dangerously close without experience. Also, he is all skill shots, which will also increase your skill in that area

Corki, because he is trickier than Ashe or Graves.
Valkyrie is a great escape and closing tool. Learnign to time his rockets adds up to loads of damage.