Thankfully, I think Bachmann is too controversial to take the Republican nomination. Right now, it looks like a tight race between Gingrich and Romney with Ron Paul in third place but slowly and steadily catching up in the polls. (Some polls have even placed Paul as second ahead of Romney in Iowa.) I think Perry has bungled enough that he is no longer a realistic contender.
Since Obama most likely has a lock on the Democrat nomination (the incumbent usually takes it for their party), it will probably come down to Obama vs Gingrich/Romney/Paul by November.
On the Republican side, Paul is the one least likely to try to pass some new form of DOMA. Take that for what you will.
(FWIW, I have no loyalties for either of those two parties. Just summarizing what I see.)