Wow, there have been alot of them, I would have to say it would be a tie between Stone Cold winning his first world title from Shawn Michaels and the Hell in a Cell match between the Undertaker and Mankind.
The Midnight Express/Road Warrior Scaffold match was a classic for sure. How about the Barbed Wire cage match between the Fantastics and the Sheephearders? Man do I miss the old territories.
I don't know if this qualifies as a great moment, but it's a memorable moment (I still remember it after 30 years) is when the Iron Sheik beat Bob Backlund (sp?) for the WWF Championship.
Our Embassy had been seized by the Iranians, our citizens were being held hostage, and an Iranian was now the world champ telling us:
Iran number 1
Russia number 1
USA ptooey
As I say, it may not be considered a great wrestling moment, but the American losing the championship to an Iraninan was memorable for me.