I am an NBA fan from way back. From Walt Bellamy and Cazzie Russell and Bob Lanier. I love basketball and the NBA allstar weekend, especially the dunk contest. And I have always thought that Vince Carter (as much as I disrespect his overall game) gave the greatest dunk contest performance ever back in 2000. You can see that show here (the last dunk is the best).
Then I watched the 2008 contest (not live last night but this morning courtesy of TIVO) and WOW!! I now say Dwight Howard's performance was the greatest dunk contest performance ever. All the dunks were mindbogglingly hard, creative, and executed with real style. His Superman throw-down (be sure to watch long enough to see the slo-mo replays) was to my mind the best of his three.
Vince still has the best dunk ever done in a game though, you can see it here.