This would be late by a couple of weeks, but Grand Theft Auto 3 gets my vote. No game ever gave me the same sense of freedom or just really had that sandbox feel before this game had come along. The slick car chases, the corny story with it's oddball cast, punctuated with an ending that had me laughing, gun battles aplenty. GTA3 was an unexpected triumph in transition; it left it's "last gen" forefathers in the dust and just dazzled me in practically every way. Oft imitated, played to death by myself and I'm sure many others, it really showed what the "sandbox" era had in store, and it really was a leap forward in gaming as a whole.