Brock Lesnar is without a doubt the best fighter in WWE. He actually trained as a MMA fighter for 2 years and competed successfully in UFC. And as far as wrestling goes, he had over 100 wins and 5 losses as an NCAA wrestler and won the NCAA heavyweight championship in 2000. All these other guys on the roster went to "Wrestling schools", light on the practical wrestling, heavy on theatrics and showboating.
Put any of those clowns in a ring with him (HHH, CM Punk, Cena (LOL), Orton) and they'd be lucky to last 3 minutes, 2 of which they would spend running around the ring just to avoid grappling with Lesnar.
Having said that, it was stupid to break that streak to a part timer like Lesnar who might be there for another year or two, instead of a workhorse who has been there for several years and helped build the company - someone that Undertaker could help put over so that WWE can start developing a new generation of superstars because their current one sucks and they are still coasting on the coattails of old farts from the 1990s and the Attitude Era.