Do you think it beats Hogan's heel turn from Bash at the Beach in 1996? Because I'd argue it does.
I've never seen a crowd as stunned as when Undertaker lost, in sport or in theatre. Nobody gave Lesnar a chance leading into the match, and expected it to be a routine WrestleMania crushing at the hands of Undertaker. When the ref counted the three, there was just silence - I couldn't believe what I was watching. You have to hand it to WWE, as they certainly did the right thing when holding off Lesnar's music for a minute or so after the pinfall.
Hogan's heel turn had a much greater impact on the business, though viewing both events in isolation, I'd say Undertaker's streak ending was easily the more shocking moment.