Hard to narrow it down, but I really had a good time at these events:

Indianapolis 500
POWRi midgets "The Battle at the Center"
Three day midget-sprint show at Tri-City (Pontoon Beach / Granite City)
Eldora USAC Three Crown
USAC Silver Crown at DuQuoin

I can't leave out the POWRi midgets at Belle-Clair or Hut Hundred at Haubstadt, as well as just about any of the sprint car races I attended there