Some of my picks:
Bret Hart vs Steve Austin - Wrestlemania XIII. By far and away the finest match I've ever seen. One of the few matches I'd truly consider a work of art.
Bret Hart vs Shawn Michaels - Survivor Series 97. Arguably the most famous match of all time, for the wrong reasons.
Mankind vs Undertaker - King of the Ring 98. Made Mick Foley a superstar. Biggest Holy-Shit moment ever.
Shawn Michaels vs Razor Ramon - Wrestlemania X. Wasn't the first ladder match, probably no longer the best, definitely not the most spectacular, but the one that put the concept on the map.
The Rock vs Hulk Hogan - Wrestlemania 18. Godawful match, but well worth watching for the incredible reactions from the crowd and the ultimate reminder of Hogan's ability to manipulate the crowd.
Travis Tomko vs Stevie Richards - Unforgiven 2004. Godawful match, but well worth watching for the incredible non-reactions from the crowd and the ultimate reminder that not everyone gets to be like Hogan. Watching this match makes you feel the same way you felt watching some of the excruciating moments of the UK version of The Office. Except that instead of laughing afterwards, you feel the need to take a series of cold showers.