Mine's probably a tie between the National Championship game (does that count as the 2002 Fiesta Bowl or the 2003 Fiesta Bowl?) and the 2004 Michigan game. The NC game is for obvious reasons, but the Michigan game in 2004 was great for me, personally, because Ohio State had a terrible record against Michigan when I was at the game. Check this out:
1986 Ohio State lost. I don't remember what the score was, but I seem to remember Matt Frantz kicking a last-second field goal that would have won the game. I may have that fact confused with another game, but I know the Bucks lost that game.
1988 Ohio State lost. Cooper's first year. The Bucks were 4-5-1 at that time. They were down something like 24-0 at halftime. They came back and made it a game. They took the lead, and the two teams traded leads a couple times, before Michigan decided to win.
1990 Ohio State lost. I don't remember much about it. Is that the game when Greg Frey attempted an option on fourth down on the winning or tying drive?
1992 Tie game. Michigan had already clinched the Rose Bowl with a tie the week before (to Illinois, maybe). Ohio State was down 13-6 when Herbstreit threw a touchdown pass. They forced Michigan to punt, then OSU had to punt. The Michigan returner muffed the punt and it looked like it was going right for the Ohio State player. Tovar, maybe? But Michigan got it back. They ran a couple times to run the clock out before trying a hail mary. Ohio State intercepted with no time left, and returned it a long way. There was some call that I remember thinking the refs missed - some penalty that should have let the Bucks have one last play with no time on the clock. But it wasn't called, and the game ended.
2000 Ohio State lost. The score was pretty close, but I don't think the game was all that close.
2004 Ohio State won (finally). 18 years after seeing my first Ohio State loss in the 'Shoe, they won one.