Recently, I'd say seeing Brock Lesnar return and beat the crap out of Cena.
But in the History, I'll go with the return of the Undertaker. After the big Smackdown vs. Raw feud, and Randy is being held up in the ring, a Casket is carried out, and the lightning hits it. Taker pops out and just starts beating the crap out of people. That was when I was a huge Undertaker mark, and it was the coolest thing I ever seen him do.