From an historical perspective in 2015, Undertaker and it's not even close. The guy was one of the cornerstones of the biggest pro wrestling company and was (and you could make the case that he still is, but I don't really buy it) the cornerstone of the biggest pro wrestling show in the world. The titles, The Streak, all of it against a guy who has had two great runs broken up by walking away at the height of his popularity to purse an NFL career followed by a UFC career. That's why Taker got my vote.
Having said that, ask this question in 10-15 years and the answer could be very different. Lesnar has a unique opportunity given his backstory, qualities, personality, abilities, etc., to become the "next big thing" once again. Lesnar has everything it takes to be the next Undertaker. And that could make him just as big a force in the history of WWE and of pro wrestling in general as the Dead Man.
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