According to ESPN, the two front-runners for Manning as of today are the Cardinals and the Dolphins, with the Dolphins in the lead. Both are nice, warm-weathered cities to live in. Although Miami may be nicer, Arizona has a domed stadium to play in, something he's used to. Although both teams are only decent in overall talent, Manning is used to that (look at the Colts), and both have amazing #1 WR's: Fitz & Marshall. And, both would be open to signing Reggie Wayne, Manning's favorite target. While I'd rather see him go to Arizona, ESPN is thinking Miami. Besides the reasons listed above, Manning actually referenced the Dolphins in his press conference, calling it a "great organization". Miami was the only team he named. Manning's very calculated and intelligent- he's not going to be throwing around comments, all of his thoughts are careful and deliberate.
The Seahawks, Chiefs, Broncos, Redskins, Jets, & Browns (who are most likely going to get RGIII) won't get him for many different reasons, some overlapping: some have QBs in place that may limit their bidding, some are in very undesirable cities, some are just bad teams with not much hope right now, and some (cough JETS cough) are having problems of their own- why would Manning want to enter into that?
It's most likely going to be the Cards or the Dolphins, and it's looking like the Dolphins.
Edit: One more point in favor of the Dolphins: They're a team that's entering a new regime. Remember, Tony Sparano got fired, and Joe Philbin is the new HC. It's much easier to create something new with a new, incoming coach than it is to come in and try to fit into someone else's established system. In other words, since Philbin will recraft the Dolphins anyway, Manning would certainly be allowed to inject a lot of influence into the new system while it's being implemented. That certainly has to be another attractive thing for Manning, as he's been known to be a control freak, and had a lot of creative control over the Colts' offense. He could have a large say over this new offense in Miami- that has to be another plus for him.