Tanks are in a really nice spot right now, so the 'best' ADCs are the ones that have good scaling into late game. Assassins that would normally be able to easily punish these immobile carries are no longer the most desirable picks in the current meta, because tanks can just walk right over them.
In one tier below that, are Graves, Lucian, Corki and (kinda) Urgot. These champions are still reasonable picks, because they have SUCH a strong bullying presence in the early game, and they often have mid game power spikes that if used well, can cascade their power into a significant item advantage in the mid game.
You might notice that I didn't include Twitch, Vayne, or Tristana in the 'best' ADCs list, even though they fit the qualifications for a desirable ADC in this meta. These three champions are still strong ADCs, but their laning phase is so weak, that it's incredibly easy for them to just get rolled over.
Kalista is the goddess of kiting because of Martial Poise! Sivir is the goddess of farming because of her kit that clear minion waves! Kog'Maw is the god of boom-booms for it shreds tanks! Jinx is the goddess of destruction for she wrecks enemy teams!
I would add Corki to this list. He's still good (like Graves), just not quite as strong when you play against really tanky teams. In soloqueue the tank meta isn't quite so dominant so he's probably still fine there.
Actually, Urgot's damage is super high once he gets a Muramana - Kinda like a blue build ezreal in a way. He is primarily picked mid, although two Noxian Corrosive Charge + Acid Hunter combos alone can force an ADC/support out of lane, so his
While he's mainly played mid, he's seen play bot in competitive play. With a decent tank line, his ult's pretty good at cleaning up fights - Most people initiate with his ulti though and you usually don't want to do that.
Urgot top and mid are more popular, but if you have an alternative source of high damage on your team, then he functions in the bot lane. I like to think he's good for the same reason Skirmisher's Sabre is good, for his passive. Also, Hyper-Kinetic Position Reverser should be used in a similar fashion to Infinite Duress. You need to be much smarter with it's usage though considering Urgot's role.
Also, I generally wouldn't recommend Kog'Maw in solo queue. He needs a lot of coordination to be the terror he is in competitive games. Do pick him if you have a lot of trust in your teammates though.
I Would say that Vayne is pretty good because her W does % of current healt and thats good against tanks in current meta, but if u ask best adc Vayne have low range and hard laning phase...
It really depends on the team comp the enemy is running. Sivir is a great pick. She has the best waveclear of any ad carry in the game has a very nice ult which buffs your team and although she's not a hyper carry she still scales fairly well into the late game. With cinderhulk being so strong hyper carries are becoming increasingly popular. Jinx and cogmaw are the most popular...cogmaw needs a lot of protection which brings us to the "juggermaw comp" which is pretty much buffing the adc so much he can't die while he does tons of dps. We can't forget Graves corki or kalista though. Urgot I'm not so sure about. I would say he's a better mid laner BUT he can be flexed over to an adc if he has to face chogath or something.
Hypercarries are becomming popular at the moment due to the Cinderhulk meta, but if you're looking for a consistent ADC I'd recommend either Caitlyn or Corki.
Both have some form of mobility Caitlyn 90 Caliber Net and Corki Valkyrie and have good poke, range and damage. They're both quite good in lane and have synergy with a lot of the current peel supports being played.
In my opinion the strongest ADC at the moment is Kog'Maw because of the extremely popular Juggernmaw team comps and because of the extremely overpowered tanky jungler's building Cinderhulk.
In my experience, the most generally powerful Marksmen is Quinn. It may have been circumstantial, but every time I've landed against her, regardless of support differences, she always tears me apart. I'm not sure if her damage is too high or her attack speed is too high, what I am sure of is that she never fails to ruin my early game easily.
I think that's only because she has a blind and decent burst which can be good in lane (vs a squishy adc). But beyond that, its risky continuing into mid/late game with her.
The 'best' ADCs right now are probably:
In one tier below that, are
You might notice that I didn't include
Hope that helps
While he's mainly played mid, he's seen play bot in competitive play. With a decent tank line, his ult's pretty good at cleaning up fights - Most people initiate with his ulti though and you usually don't want to do that.
Also, I generally wouldn't recommend
Both have some form of mobility
Other than that, there's really not that much.