It's so hard to decide...
Queen, with Freddy Mercury, perhaps the greatest live performers ever, (with Paul Rodgers, I don't know what the fuck is going on there, but it sucks..)
Black Sabbath & Pink Floyd were the most innovative bands of the '70's
Lynard Skynard and The Allman brothers defined southern rock
Led Zepplin brought rock's blues background to the forefront of modern music
Metallica brought metal out of the garage and made it a serious contender in the music industry...(and thankfully, put an end to the Hair Bands..)
It's like potato chips, you can't stop at just one...