Either Undertaker's or Ric Flair's... Hmm... I'm going with The Undertaker because he's undefeated at WrestleMania. Flair's accomplishment means that he also LOST 16 times in championship matches, while The Undertaker is undefeated at the biggest wrestling event of the year.
I agree with almost everyone, I had to go with Undertaker winning streak at the biggest stage of them all and that's Wrestlemania. Then you have Stong Cold coming in at a close second, then you round up third place with Goldberg winning streak.