I love me some Hamm, but I just don't think he's reached the point where he can be considered A list and People won't give it to someone who is not a Movie Star yet. Plus he is on TV, but Mad Men, 30 Rock, and SNL don't have big ratings. There are just too many people who don't know who he is yet (outside of the gossip and entertainment worlds), so while he gets my personal vote, I'd say it'll be RDJ, or maybe Pitt because they're going with a three-peat in honor of the 25th year of the title.
Now when is Hamm going to break out in the movies? He's totally the next Clooney.