Oh people actually think Trist is good now? Okay I can shamelessly admit how powerful Tristana is. IMO Caitlyn and Draven stand at the top of the table, the former having the best early game presence and pushing power in Piltover Pushbullet while the latter is simply a powerful aggressive laner with great mobility to scurry around to quickly reposition (which most ADCs cannot). 
Tristana is the best to me because I suck at using anything else and her W and R let her peel for herself. Also, late game her insane range and constant W refreshes make her nigh unkillable while granting the team powerful tower pressure.
Miss Fortune joins them in the major leagues for her ability to deal massive AoE damage in Bullet Time, which allows her to fire off the bulk of her damage very early in the team fight, making certain AoE team comps highly effective (Malph, Orianna, related champs). 
Twitch and Ezreal are two highly odd ADCs that play somewhat differently. Ezreal bullies as well as, if not better than Draven, and his mobility, farming and wave clear is stupidly good. Twitch is so powerful against unprepared teams, and essentially beats and ADC 1v1 midgame. His Expunge is a menace to society, with a well-time Spray and Pay granting him enormous burst damage. 
I almost forgot Vayne, who snowballs far faster than any other ADC and simply ravages bruisers even without her Last Whisper. Her ultimate allows her to entry the fray and spread confusion amongst the enemy team.
And now we move on to the shitty ones.
Sivir is lol. Meh in general, with no notable advantages over other ADCs besides perhaps her quick lane clear and being Blitz-resistant.
Ashe sucks. Slow as hell and prone to ganks. Her global ult is good, but unless she fires it off in fog a coordinated team is going to know when she fires it and where.
Quinn has great burst, actually, but to achieve this she must put herself in harm's way, and her ult basically turns her into Master Yi (minus the bulk).
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I reserve comments on Graves, Corki and Kog'maw, given their low usage despite theoretically being highly effective in what they do. Graves has pushing power and a strong ult, Corki has high teamfight damage while Kog puts a lot of pressure on towers and contributes a lot to a poke comp.
Oh and Varus is a turd I hate him