Here in England, it was bands like Oasis, The Verve, Pulp, and the Manic Street Preachers who really defined our musical decade, at least in the mainstream. The Manics were probably my personal favourite, simply because of how well-written their songs were, and how awesome their visual imagery was.
Going a little bit more towards the heavier side of things, I'd say Pantera, Marilyn Manson, and Slipknot were all pretty definitive. Pantera carried on the 80s metal legacy, albeit to a far heavier and brutal degree than previously. Manson made rock and roll genuinely dangerous and shock-inducing again, and when Slipknot came onto the scene, masks and all, that was something isn't easily forgotten. Watch the music video for 'Wait and Bleed', and you'll see what I mean.