If these reports are true then that rules out 16 teams automatically just because they are in the NFC. Also you can eliminate team like the Patriots, Colts Steelers etc. for obvious reason. The Dolphins, Jets, Broncos and Titans if all the reports are true. I can't see him going to the Jets and Broncos because I think it would bring too much drama and the Titans don't seem like a likely scenario.
From all the reports that I have read I have come to the conclusion that I seriously think the Dolphins are the favorites. I could be wrong, maybe the Cardinals make a strong case and Manning disregards the fact he could play against his brother to reach the Super Bowl. But it just seems like the Dolphins make so much sense for Manning and the only downside I can find, if this is one, is that he would have to play the Patriots twice and we are in a fairly hard division. However Peyton is extremely competitive and could always make that a plus instead of a negative for the Dolphin's chances.