I'd say kog'maw. He does require a solid team comp for him, but GOD does it work well. Plus, he's got fantastic range and has a better time killing tanks/bruisers compared to twitch. Twitch has the AOE, they both have great range, but i'd say Kog'maw is better with full build.
Tristana's definitely up there, as a hypercarry and all, but i'd still say vayne or kogmaw are better lategame, since most enemies will have gotten a large amount of hp.
But this all assumes both teams are a fairly standard comp. Against a 5-man assasin comp? Any adc is screwed :P --- Idealism is seeing potential. Pragmatism is seeing consequences.
Although Kog'Maw definitely has amazing full build carrying potential, there's really no denying Tristana's escape, range, and knockback. I think her and Vayne are comparable with full builds but I would say Tristana > Kog'Maw simply because of her escape options. --- c|(:B)--|--< Howdy ya'll!