My top 10 and why...
1. Pink Floyd - There has not been and will not be another band with this sound. It's Rock Chamber Music. Their music has been covered by others, and adapted by orchestras, but the original recordings by the band still stands on it's own as the defining version every time.
2. Peter Gabriel - The most original solo artist of the past 30 years IMO. Career has explored Rock Opera, World Music, Acid Rock, Orchestra based, and a vocal style all his own.
3. Led Zeppelin - Who else has so perfectly married Blues and Rock together?
4. Grateful Dead - The antithesis of perfect fusion of multiple musical styles, followed closely by....
5. The Dave Matthews Band - ...another successful fusion of different styles of music.
6. Silversun Pickups - Best new band of the past 5 years and if they can keep it up, they'll bump Smashing Pumpkins as best alternative band ever.
7. U2 - Great music with an important message to back it up. Defined my high school generation.
8. AC/DC - The best band ever to produce 35 years of 3 chord songs that sound almost identical from one album to the next, but never get tiring.
9a. Van Halen (pre-Hagar) - The second best band to product non-boring 3 chord songs.
9b. Van Halen (Hagar era) - A great evolution of a great rock band, and never get's boring for me.
10. The Alan Parsons Project - An amazing story in music history. A band completely made up of low paid studio musicians, hence the "Project". Listen to the remixed version of "Tales of Mystery and Imagination" sometime, and you'll hear one of the finest concept albums ever made, and the narration by Orson Welles just makes it that much better. The most underrated band of the 70's/80's IMO.
Sorry for the edit, but honorable mention goes to...
11. Pearl Jam - From the ashes of Mother Love Bone came a band that was angry, powerful, and important, and still commands respect today even though the days of the "Grunge" movement are over.