Probably Frank Mir and Big Nog. Mir had a serious motorcycle accident, struggled to return to form but eventually did. While he only had one big comeback win which was the second Nogueira fight, climbing back to the top after the accident and then the late career resurgence he had are impressive.
Big Nog was run over by a truck and nearly killed as a child, but that didn't stop him from becoming PRIDE heavyweight champion and UFC interim heavyweight champion, and he probably has more comeback wins than anyone (and they are all dramatic).