Tarver had a much closer fight the 2nd go around, starting in the mid rounds. He was simply too old to execute. Other candidates: Joe Louis Lennox Lewis Wlad Klitschko Muhammad Ali
A lot of "rematch specialists" are just fighters that needed two fights to solve or become dominant over lesser talented fighters.Or lazy *******s that tended to take opposition lightly. That's not always the case, but generally i prefer fighters that don't need two fights to think on their feet.
I'd say Evander Holyfield can get a mention. He's not quite as consistently good in rematches as other fighters but has won some big fights in rematches. He had that war with Dwight Qawi the first time around, then stopped him in 4 in the rematch. Won the 2nd fight against Riddick Bowe. Avenged the loss to Michael Moorer with an 8th round TKO in the rematch. Was certainly more competitive against Lennox Lewis the 2nd time around, and some felt Holy deserved a win or draw (I am definitely NOT one of those however). It's been years since I saw the rematch with Alex Stewart, I remember it being boring after the first fight was good so I don't know how this compares. Other rematches were the infamous bite fight, and the 2nd and 3rd John Ruiz fights. The 3rd match against Ruiz, a god-awful fight, was probably an improvement for Holyfield over the first two.