There's a couple of ways to answer this question so I'll cover it from all angles.

The coolest event I ever attended was the IU vs. Duke ACC-Big Ten Challenge matchup my freshmen year in 2005. Duke was #1 in the country with Redick and Shelden Williams and they were expected to blow out IU. I had seats in the 9th row behind the basket. When Marco Killingsworth had the breakaway dunk with about 6 minutes to go to put IU ahead for the first time Assembly Hall went absolutely nuts. I remember jumping up and down on the bleachers and falling all the way down to the low part because everyone was out of control. The ending result wasn't great because IU lost, but that kind of atmosphere was never matched...although the victory over Wisconsin when they were #2 and the ESPN College GameDay drubbing of Michigan State both were close.

The coolest event I've ever played in is a tougher question. I got to play in Assembly Hall twice for Intramural Basketball Championships (winning 1) so that was a pretty cool experience although neither game was great. We did have some pretty awesome intramural games before that in basketball, flag football, and ultimate frisbee. My 2 favorite games that I played in (mostly because of the results) were the Interns vs. Full-Time game when I interned at GE and my intramural basketball championship game my senior year of high school. The Interns vs. Full-Times was an annual event and the interns had never won. We beat them by 20 even though they still probably had the more athletic team. The high school intramural championship was great because we played a team with a lot of our friends so there was a pretty big rivalry going on leading up to the game. They played zone so we shot 3's and ended up making 18 of them for the game. I made 7 3's including one from the corner that somehow managed to hit the rim, then the backboard, and then the rim several more times before falling in.