Mike Tanier presents research that suggests that Dan Marino might be. I hope the nail down the criteria on this. We've looked at Manning's great wins and classified them "4th Quarter wins" instead of comebacks, because in some cases the game was tied. By my informal count, just from the games on that list (which is not meant to be exhaustive) I count 20 come from behind wins in the 4th quarter or OT. That counts last year's Jacksonville game in which the Colts scored 17 fourth quarter points to win, even though the last 7 were scored by the defense on a pick. The Colts started the quarter trailing 24-14. Manning rallied the team for 10 points to tie the game before the D sealed it with a pick six.
I wouldn't be surprised if Manning owns this record too before all is said and done.