Taking the emotion of teams I root for out of the equation, I'd probably go in order something like:
1. Miracle on Ice
2. Game 7, 1991 World Series
3. Superbowl III
4. Olympic Gold Medal Game - Men's Basketball July 5, 1992 (just to see the Dream Team)
5. Ohio State v. Miami January 3, 2003 Fiesta Bowl/National championship (granted I am an Ohio State fan, but a double OT thriller with a controversial penalty for all the marbles has to rate up there as one of the best college games of the last 40 years even for non-Buckeye fans - USC/Texas would also be high on that list)
After that I'm not sure. I would have loved to have seen Phelps win his 8th Gold Medal of the Beijing games by a hair. Seeing Usain Bolt crush the field to dominate the 100 meter dash. There have been any number of great World Cup soccer games over the years (same with Superbowls, I chose Superbowl III because of the massive historical impact though I'm sure there were better games).