When I see the word "comeback", I feel like people who were rookies last year shouldn't count... because they aren't really having a "comeback", they're just developing and improving. Comeback implies (at least in my opinion) that they previously had 1 or more good/great seasons, fell off for 1 or more seasons, and beat the odds to return to awesomeness.
Spencer, Jenkins, Barbie, etc have definitely improved this year, but I'm not sure if I'd say that any of their previous seasons were considered "good" in order to warrant this season as a comeback. The player that best fits that description IMO is Kosier.
EDIT: looks like ABQCOWBOY beat me to it...