In the last few years, most definitely Piper appearing at Wrestlemania XIX. It was the one thing that I hadn't seen even alluded to on the internet in the days leading up to the event.
Now then, in my pre-internet days, Piper as the guest referee at Wrestlemania X was nuts, as was the Ultimate Warrior appearing at Wrestlemania VIII. Shane McMahon appearing at No Way Out 2000 was highly unexpected also.
As far as minor stuff goes, the Honky Tonk Man at the 2001 Royal Rumble was a pleasant surprise.
So far as moments as opposed to events, Eddie Guererro coming to the ring after Benoit won the World Title last year at Wrestlemania XX was insane. Nothing I've seen since and only one thing before that has had the same raw emotion involved. Only the Savage/Elizabeth reunion at Wrestlemania VII trumps it.