Caitlyn
Good for dominating in lane because of her passive and long range, but to me playing her gets boring after a bit. She can be built as attack speed because her passive gives her more damage, but it's a risky move to make in Ranked.
Varus
Extremely good poke with his Q, but his combo requires Basic > Basic > Basic > Q or E (this is without his ult) which can be a bit difficult to pull off early game if you don't have a good amount of attack speed.
Vayne
Vayne can always get back into the game because of her good scaling. No matter what, she will always do insane damage if played and built properly.
Kog'Maw
Kog'Maw is a known hypercarry - this means he can pull back into the game very well just like Vayne, and will also always do a lot of damage. His ult allows for low-health opponents to be picked off easily over a long range. His passive is great in team fights, but don't always expect to get a kill.
Sivir
Sivir uses a lot of mana, so you really have to use your abilities wisely. Her spell-shield is great for blocking ults such as Darius's ult, Karthus's ult, and other execute ultimates.
Jinx
Jinx can really put in a lot of damage per second if you build her right and max her Q. Her combo usually requires 3 basic attacks to get her Rev'd Up Q passive out to get bonus attack speed. Her Chompers are great for pressuring lanes, and most people aren't smart enough to step on them when both Jinx and her Chompers are under tower because the tower will target her. Her W and E are great peels, and her W is great for chases. Her ult is also good for finishing off opponents long-range and close-range, but of course, as a Jinx player, you probably know this.
Graves
Does so much damage at any point in the game, but isn't very good for attack speed because he's more of a burst ADC, unlike Jinx. His ult is good for initiates and finishers because of it's long range and high burst. His Q is also great for poking, team fights, skirmishes, and 1v1s, his smokescreen is a good enough peel and keeps his opponents from attacking him temporarily.
It really depends on your playstyle, so all of these champions I'd recommend to any ADC, so just pick as you will.