I think it's possible to rank the ADCs. The arguement saying "each has his niche" is true, but it's true for every single hero in a competitive MOBA (at least, it's the concept). And you still see tier. Unless everybody agree that all the ADCs are at the same level (which is obviously not the case), you can build some TL.
So my opinion :
S Tier : Varus Caitlyn
A Tier : Miss Fortune Ezreal Draven Twitch Vayne
B Tier : Corki Kog Maw Tristana Sivir Graves Urgot
C Tier : Ashe Jayce Kayle Kennen
D Tier : Twisted Fate Quinn Teemo
So, I ranked Varus first, I wasn't sure about Caitlyn in S, but there isn't much difference between a (X) Low Tier and a (X-1) High Tier. Her lane control is unmatched, and the fact that it's probably the safest pick is a big advantage.
Miss Fortune for her damage output was close to S. Not much to say about A, Twitch is a really competitive AD at the moment.
B Tier, I feel like Corki is really fine, but not played because considered as "outclassed" by Ezreal. In fact, by the past (early S2) NO ONE was playing Ezreal because he was considered as outclassed by Corki. Those two have an intense competition going on.
Kog Maw is a key ADC in my opinion, I may have ranked him a bit high but to me he's really better overall than Trist and his main downside right now is the fact that every single tank is building a Bulwark, so he can't really melt them as before. His range is just insane and with good positioning you get rewarded far more than with a Trist (but you don't have this escape, however your late is stronger).
Sivir is HELLA underrated, she does really fine against most of the top picks minus Draven and MF. Out of that, you will suffer a bit from the range, but she is really a nice AD that people should try more.
Graves is "ok" but outclassed by MF in my opinion, and Urgot is an ADC that nobody plays, he can be really fearsome.
C, Ashe has her ultimate and the orb walking kiting. You sacrifice a lot of burst and early game/laning power for that.
Jayce is.. well "decent" and can be really good if paired with the right support, Kayle is really playable and self sufficient as AD, Kennen got those stuns that every single AD is begging for. Those ADCs have REAL niches and "can" shine even if they are mostly outdamaged by the more classic ADCs.
D, to me they are barely viable. Even if, TF AD is something I tried (yes, in ranked) and it works "ok". It's not the best thing ever, but the stun is really cool and his ability to gank mid and the jungle is valuable. I feel like he's outclassed by Twitch here however.
I'm not gonna talk about Teemo and Quinn. To me Quinn should be played as mid given her mobility, as ADCs she's just bad and I rather have a Kayle able to protect someone and deal relevant damages, or an ADc with real CCs.
Oh, and this is my personal list, not something official or what, so no need to go crazy because Trist isn't A. She doesn't deserve it anyway rofl, Corki is just better :D