With me, my personal list keeps changing depending on the type of music I'm listening to. I've gone through phases where I was really into Motown, the Blues, Irish music, 60's music, a punk/new wave phase, and even a country/cow punk phase.
One question would be 'backing' bands. The Heartbreakers are a backing band to Tom Petty. the E Street Band is a backing band to Springsteen. Do you rate those groups the same as you would a group like the Beatles, the Stones or the Who?
but - some of my personal favorite groups, in no particular order:
Queen
The Band
The Who
Allman Brothers
the BoDeans (before Sammy got kicked out for being a perv....)
the Clash
the Alarm
The Grateful Dead
Blues Traveler
Pink Floyd