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A lot of people aren't sure how to describe or pick one, so it doesn't have to be just one. My favorite band is definetely Rush, runner up is Led Zeppelin.
Well I could classify them as best in various categories:
Highest % of masterpieces among all their songs: Pink Floyd
Highest skill with all instruments: Dream Theater
Best live performances: Pink Floyd
Most melodious without sounding gay: Led Zeppelin
Most meaningful: Bob Dylan