1) The Grateful Dead
WHY: The Dead are single-handedly responsible for two indespensible parts of modern music: close mic technique, and massive sound systems. Before their 'wall of sound' system in the mid-seventies (only 80 concerts, but WHAT concerts!), bands toured and used whatever PA was in the concert hall at the time. I mean *ALL* bands. This made for really uneven sound quality. You could rarely hear concerts the way they are now. Close mic technique is another of their techniques, and it's a way of placing mics on drums & amplifiers for better sound reproduction live and in live recordings. The difference is one you couldn't imagine if you haven't heard it.
2) The beatles
WHY: Everything that the Dead did for live music, the Beatles did for studio music. Before the Beatles, bands went into the studio at the whim of their record company; the company decided when, what producer, how long, and -- get this-- what songs. The record companies were in complete control.
3) Led Zepplin
WHY: I'll leave the answers in this thread stand.
4) The Ramones
WHY: They broke down corporate rock and roll in the '70s.
5) KISS
WHY: Rock music as theatre.
6) Bob Dylan
WHY: The single most-covered musician in all of rock 'n roll. Somehow, nearly every band can find something that Bobby wrote and turn it into a song of their own. Add in the fact that he was the spokesman for his generation (although he didn't want the attention), and you've got a solid position on this list.
7) The Who:
WHY: Although The Ramones were responsible for popularizing American punk, the Who were in many ways the original punk rock band. Add in their astounding innovations in terms of rock opera and rock film, and you've got something special.
8) Jimi Hendrix
WHY: I doubt if any guitar player has shook up rock like Jimi did. His playing was so crazy, so far beyond anything that anyone else was doing, that you just can't get beyond him.
9) Chuck Berry
WHY: well, he was the first (or at least the first popular) guy to do what he did, and what we know as Rock.
10) Janis Joplin
WHY: she was the first rockchick, although if there's a better example from earlier, I'd be happy to replace her. She was amazing, though, and her live performances are a treat, if you ever get a chance to see a recording. She's so alive. More interestingly, she'd NEVER get a record contract today, given her acne and the fact that she wasn't pencil-thin. Like those things matter when it comes to music.
11) Frank Zappa
WHY: There are a few interesting things that make FZ so interesting. He was aiming for using classical composition techniques in rock music, which is somewhat unique. His use of humor is pretty great. He had an 'open' band-- anyone could come and audition to play for him, and if they were the best, then they were in. This is how we got to meet Steve Vai.
12) Chicago
WHY: This is an emotional favorite for me. My dad went to college with these guys, and was around when they said, 'hey, let's put together a bar band with horns'.
13) Bob Mould
WHY: This is an esoteric one, I'll admit. Bob was the guitar player for Husker Du, an '80s punk band from Minnesota. He was probably the most virtuostic guitar player in all of punk rock. Husker Du was also one of the most musically advanced punk bands ever. They were punk, but they had technique, and they could improvise as much as any jazz group.
14) King Crimson
WHY: King Crimson were the prototypical 'prog rock' group. Between Adrian Belew's crazy "anything goes" guitar (he once played a guitar with a floppy rubber neck, just because he could), and Robert Fripps' almost mechanical precision, they had some really crazy and competent sounds. Bill Bruford was an incredible drummer. Every prog rock band since owes a lot to them.
15) George Clinton and Parliament/Funkadelic
WHY: Although these might belong in R&B/Hip Hop, I think that George and company changed the funky side of music forever. They had amazing live shows, and an unmatched volume of output in the studio. George and his crew had contracts with every record company, and would put out one album after another, switching band names as needed to stay within terms of their contracts.
16) U2
WHY: U2 refined a lot of things that have been mentioned before on this list. I don't really have much to say about them that's interesting; I just feel that they're pretty darn important.
17) The Doors
WHY: Another sentimental favorite, their organ player was an alumnus of my high school. Funny thing about the Doors-- they had no bass player. The organist took care of it. On nights when Morrison was too drunk or high to sing, he would take that over, too.
18) Nirvana
WHY: They burned brightly, and for a very short period of time. They were influential in their own small circle. A million garage bands were born under their influence. I think it's maybe a bit early for me to know if they really belong on this list or not.
19) Eric Clapton & Cream
WHY: I'm almost embarassed to have him this low in the list, but Eric's blues influence has been felt for years in rock music.
20) The Rolling Stones
WHY: They're the rolling stones. Any list would be incomplete without them. They weren't the best at what they did, but they were good enough. Sometimes, that should be all it takes.
Wow, that was long winded.