GTS looks sloppy and fake
I really have to agree with this. I know that this guy is hating on Punk, because he does so everywhere else, but I've never believed the GTS looked legit. You get this guy on your shoulders, making everyone think you're gonna do some devastating slam or drop him on his head, but instead you shove him off over your head and knee him in the chin. The move seems to fall totally flat once that happens, in my eyes. I know that a knee in the face would actually hurt, but the move itself looks extremely fake and not painful at all.
Other terrible finishers:
Everyone has already mentioned the Cobra, the AA and whatnot. I'd also like to give a shout out to Rikishi's stink face (it basically was his finisher). A fat man sticking his ass in someone else's face...it had its comedy moments but yeah, it was pretty terrible.