On personal preference, it would be: Radiohead U2 REM As for historical significance, they will all go down in the books as great bands. IMHO it's too early to rank them though, Radiohead needs a few more years to judge their impact. I applaud all of them for their longevity and their constant growth of sound. They've all put out 'risky/alienating' albums that many complained were too divergent from their respective pasts, but always prove the detractors wrong. Kind of off topic: It's occurred to me recently just how long the Red Hot Chili Peppers have been going strong. I'm not really a fan of theirs (I like some of their songs, never really followed them though), but it's pretty amazing that these guys still stay relevant. Most of their counterparts when they started out have already retired, willingly or not.
[musical snob post]That's another band I was listening to back in 1984 before anyone else knew who they were. I even saw them in concert at a half empty Sam Houston Coliseum back when Hillel was still alive. Their pre-Blood Sugar material is so much better (and less watered down) than their post-Blood Sugar material. As good as Chad Smith and Flea are, it's like listening to two different bands. [/musical snob post]