Here, of course, from my recent column for Antiwar.com, is my preference: American withdrawal from Iraq as quickly as possible. I think I went through most of the objections and answered or neutralized them reasonably well.
Here's a particularly noxious one, invoked all the time by Bush & Co: that American withdrawal would immediately make Iraq a safe haven for international terrorism. A lot of things could happen after a U.S. withdrawal from Iraq, including civil war. But whatever emerged would almost certainly not be something like the Taliban in Afghanistan, eager to provide refuge for al-Qaida and other international terrorists. In fact, given that al-Qaida is generally Sunni in persuasion and the majority in Iraq are Shia, Iraq might very well become one of the unfriendliest places places on earth for al-Qaida.
That's not the guaranteed outcome, and I have no crystal ball. But Iraq as an effective safe haven for international terrorism is one of the least likely results of a U.S. withdrawal.