Nishikori, Monaco and Karlovic have to be the leading contenders imo.
Kei had an incredible finish to 2011, and should challenge for another title in 2012, although he has committed to a tough schedule in the first quarter. Brisbane boasts a strong field, and it looks like he is playing South American clay in February?! Assuming he sees sense and commits to the likes of Delray Beach, Houston etc he'll get a better chance.
Monaco also had a strong finish to 2011, but he really struggles for consistent results. Good chance to win something early in 2012, being one of the front-runners in Auckland and top seed in Vina del Mar.
Karlovic is still a threat, and he is likely to enter enough weaker 250 events to give himself a shot.
Petzschner - talented player, but a nutter. Will threaten on grass, and in weak indoor events, but always questionable whether he can repeat that Vienna triumph.
Kunitsyn - did well to win in Russia a few years ago, but I never really had him down as title-winning material. I still don't.
Darcis - see comments on Petzschner and Kunitsyn.
Malisse, Blake, Nieminen and Hanescu are past it imo. Nieminen is the more likely to win a title out of this group, although he is limited really to indoor events, and needs the weaker draws. Malisse and Hanescu are both questionable players - mentally, physically, and as regards motivation. Blake's game is still as big as ever, but he cannot manage the same consistency as he used to. Surfaces are slower/balls heavier, and the game is more physical - both bad news for him.