If I could travel back in time to see one band and only one band, I'd have to go with Joy Division...probably in Amsterdam, at the Paradiso. I have a bootleg from that show that never fails to make the hairs on the back of my neck raise.
I'm awfully glad I got to see the Ramones, Nirvana (@ Bender Arena), the Pixies, etc., but to be honest I enjoyed the Breeders opening for Nirvana more than I enjoyed Nirvana. It was not an enjoyable experience to watch Nirvana...and more specifically, not enjoyable to watch the audience watching Nirvana. A lot of people were getting hurt, and hurting each other. Of course, that was back in the day when the moshing thing was really getting out of hand. I enjoyed Smashing Pumpkins the following week at WUST a lot more...with Swervedriver opening, no less. What a great concert that was.