Really Halo3 enjoyed the Single Player, enjoyed the 4 Player Coop Campaign pretty damn much, which wasn't very Common back in the days. Didn't Know for sure, but Maybe it was the First 4 player Coop on Consoles?? Forge was Fun as Well. Especially MP was Great, enjoyed Hours playing online.
Soundtrack is still the greatest of all Time for a Video Game. When counting the Add on Halo3 ODST to the Halo3 experience Firefight was top Notch fun