I've been fond of classic rock for a while now and would have to say the following are some of my favorites:
Led Zeppelin (I know *everyone's* favorite) - I can't stand Stairway because it seems like every crappy classic rock station has it on at any given moment but their other stuff is so kick ass. I think it's funny to look back at them and realize that they were THE *boy* band at the time. Robert Plant was chosen for the band because he looked good.
Jackson Browne - The live album Running on Empty has to be a top three album for me. Such a great musician ... nothing really needs to be said

. If you've never heard the entire Running on Empty album there a lot of great tracks that get little to no air time (Cocaine comes to mind). Cool story behind the album as well ... look it up on wikipedia.
Jethro Tull - All I can say is "wow". Who else has the balls to play flute in a "heavy" metal band? He was saying that with his lyrics and Zeppelin's music they could have created the perfect band ... and I agree.
Jimi Hendrix - nothing needs to be said
I like a lot more bands from this time frame (Doors, Beatles etc) just too many to list :-p