If healthy, physically able (& motivated) -
1. Hasselbeck
2. McNabb
3. Palmer.
Palmer is the most intriguing because he figures to be the youngest and most physically gifted. But you have to wonder about what he's got going (or not going) "between his ears."
Just wondering, but does the fact that a bunch of teams drafted rookies like Newton, Gabbert, Locker, Ponder, Kaepernick etc. - open the door to the likelihood that some of those teams will be more likely to release or trade pretty good backup QB's.
For example, I've heard Hoyer (NE Patriots) is a hot commodity among competitive scouts.