Zeppelin will always be my favorite, but I've been on a huge Floyd kick lately.
Other classic rock bands that are sorely missing in the list (off the top of my head): The Stones, Cream/Eric Clapton, David Bowie, The Who, Jimi Hendrix, Queen, Rush, Grateful Dead...
To those who responded with Pink Floyd: what do you think of the early stuff? I started listening to Atom Heart Mother and Meddle recently, and they are amazing...Meddle is just so mellow...I love it!