Let's toss Rodgers out of the MVP discussion because there's enough perception around the league--fair or not--that he's having a down year. Certainly "down" enough to keep him from accomplishing a rare back-to-back.
Brady and Manning are considered the front-runners, but their numbers and team success are similar enough that Peyton's "story" and the Broncos' improvement over last year should give him the inside lane.
That brings me to my personal favorite, Peterson. In my opinion, he probably needs a playoff berth and/or the Dickerson record (mayyyybe just 2,000 would do it, but that seems like a lock at this point anyway) to have an actual chance, but I hope I'm wrong. Manning (4) and Brady (2) have won enough of these things--this is the year to honor the first RB since Tomlinson in 2006 (and Shaun Alexander before him). I think Peterson's talent is clear at this point. History will remember him as a superior RB to either of those guys. Factor in his miraculous ACL comeback, the loss of Harvin, and the uneven play of Ponder, and this performance just feels more historically consequential than what Manning is doing, and Peterson should be awarded accordingly.
As for Comeback Player of the Year, I actually think Peterson's candidacy pales in comparison to Manning's. Peterson had almost 1000 yards and 12 TDs last year. I know he "came back" from an injury earlier than expected, but Manning missed all of last year and has come back from some crazy-sounding medical crap. I know it's a pretty weak consolation prize, but this should all work out with CPOY going to Manning and MVP going to Peterson, even if the Vikings fall short of the playoffs.