I'd have to say Holyfield. Holyfield was never supposed to be an effective heavyweight. When he made the jump from cruiser back in the late 80's, most thought he lacked the power skill to suceed at heavyweight. Even after he beat douglas no one wanted to give him his due. It didn't help that he turned in poor performances against Cooper and Foreman. That only added fuel to the fire. After he lost to Bowe, people just sort of wrote him off as a sorry chapter in heavyweight history, until he regained the title in their rematch. He actually peaked in the late 90s after beating Tyson. He lacked the power and speed to be a dominating champion, but had a great run.... By the way Ruiz sucks and hasn't accomplished anything (I don't recognize his title as legit) so he doesn't even enter the equation.
You have to remember that Wlad is trained by Emanuel Steward now, Lewis' former trainer. Wlad looked a lot more careful in his fight against Peter, despite being knocked down 3 times. He reminded me of Lewis, in that he just kept Peter at bay with his jab. I think thats the game plan for him here, and it'll work. Remember, Lewis was an underachiever until he teamed with Steward...
I went to the peter/Klitscko fight in Atlantic City a couple months ago... The scene was cool. It looked to me like Peter could definately use some conditioning. There were a couple rounds where he barely even threw a punch. With the proper conditioning, he could be a dominating force. Lets hope he doesn't turn into the next Riddick Bowe.