#2 Kane
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The Undertaker's brother Kane debuted in 1997 and immediately started a feud with him. However, in 2010, Kane defeated Taker three pay-per-views in a row.
At Night of Champions 2010, Kane retained his World Heavyweight Championship against Undertaker in a No Holds Barred match. Then, he defeated Undertaker in a Hell in a Cell match, with help from Paul Bearer. Finally, he ended the feud with his brother in a Buried Alive match where the Big Red Monster won with some help from The Nexus. The wins might not have been clean, but Kane beat Undertaker 3 times in a row, and that is very impressive.
#1 Brock Lesnar
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This will come as a shock to no one, as no one can forget that Brock Lesnar ended the streak.
In a moment that went down in history, Lesnar pinned Undertaker after three F5s and broke the greatest undefeated streak in wrestling history. That happened in 2014, at WrestleMania 30, and the pair had two other matches the following year.
Undertaker beat Lesnar at SummerSlam 2015 when Lesnar passed out, but the finish wasn't clean. Undertaker actually tapped out, but the referee didn't see it. This all led to a final match between the two at Hell in a Cell 2015. In a gruesome match, Lesnar pinned Undertaker to finally end their legendary rivalry for good.
Lesnar beat Undertaker clean twice and arguably was The Undertaker's greatest rival.
Did a WWE Hall of Famer predict John Cena's heelish ways long ago? More details HERE.