Here's my list, It's going to be fairly biased towards my favourite artists, but I am giving credit to most of Rock's essential innovators. I've based my list on a combination of my perception of an artist's talent, showmanship, lasting popularity, innovation, creativity, originality, and influence on other artists. I've also added in some side notes.
1) Led Zeppelin
2) Rush (Huge talent, rarely awknoledged for their achivements)
3) The Beatles
4) The Jimi Hendrix Experience
5) Pink Floyd
6) Elvis Presley (Possibly the single biggest influence on rock other than the Beatles)
7) Black Sabbath (They popularized metal)
8) Van Halen (Most of the 80s was spent trying to copy Eddie's style, and they were a very solid band)
9) The Who
10) Frank Zappa and the Mothers of Invention (One of the most original artists I've ever heard)
11) Cream
12) The Doors (The kings of psychadellic music)
13) Deep Purple (Very underrated)
14) The Guess Who
15) Iron Maiden (80s metal at it's finest)
16) AC/DC (Possibly rock's best live band; tons of raw energy)
17) Queen
18) Aerosmith (70s Aerosmith; they lost their edge after they cleaned themselves up)
19) Dire Straits
20) Yes
I know it doesn't mean much, but honorable mentions go to Dream Theater, Bruce Springsteen, Bob Dylan, Chuck Berry, John Lennon solo, Wings, ZZ Top, Guns N Roses, and many others that I can't think of at the moment.
And on a side note, I nearly left Pink Floyd out of the list...