Ezreal, Graves and Corki aren't super dependent on specific supports, they are versatile and they do better with some supports, but they never do meh with any of them.
Vayne is okay, but you'll have trouble trading a lot and is really a position oriented champion. Even with good positioning though, you can still lose fights just because other ADCs out damage you early on. True hyper carry late game when you get enough of your items.
Varus might be okay for season 3 with new items, but right now he trades off viability because his gimmick is to shoot someone 3 times like Vayne and then hit them with a skillshot as well. Too much work to basically do the same amount of damage as other ADCs with really no benefit. Just more work to do the same thing as others.
Ashe is a terrible pick for beginners. Her ult takes the most skill to hit reliably and she doesn't do damage for a long time. She's a kiting champion in a game where you can't kite something that jumps into your face already. She has no quick escape and everything you can employ to keep yourself safe with Ashe would make other ADCs even safer.
Caitlyn would be my next best option as an ADC to learn. Longest level 1 range for an ADC, traps can keep you safe and you have net which also helps keep you safe. Her late game isn't as strong as the top ADCs like Graves, Corki or Ezreal due to the massive amounts of AOE they do in general, but unless you're playing at an absolutely outrageous level, the slight differences in power isn't enough to win/lose the game for you.
Kog'Maw is a fun champion, hard to play, but loaded with range. Only problem is he really needs Nunu to compete early on. The steroid from Blood Boil makes him strong enough to compete in lane, but without it he's only kind of average. However he is a beast and can rip people apart from a huge range away.