Peter, Castillo, and Briggs are all finished as legit contenders. Hatton and Taylor can go on with their careers if they choose to do so. Although their futures don't look very bright.
Peter and Castillo. Hatton can still go on and make some noise at 140 or 147 if he wishes to, he's not totally finished in my view just ran into a FAR better fighter. Briggs might be finished but I won't add him to that list. He's in the heavyweight division and as he proved previously one punch KO power is still good enough to make some noise/grab a title in the division. He was losing to Lyakovich and we all remember what happened there. Taylor can still go on though his desire is questionable. He definitely has the tools but the heart/will is clearly not there and hasn't been there for a long time.
I don't think Taylor's done at all. He's young enough to continue and I don't think that the way he lost to Froch or either of the Pavlik loss' gave any evidence that he's shot. To me, the answer to this question is RJJ.
Holyfield is an obvious one. Although he's been finished for about a decade. Yory Boy Campas. Don't know why he's still around getting his face bashed in. Raul Marquez returned for some unknown reason, but he's done. Andrew Golota. Oliver McCall. It amazes me how much respect that sobbing drug addict still gets.