Toss up for me between Radiohead and Pink Floyd.
Pantera*
Metallica
G'n'R
Down
Led Zeppelin
Old thread - listening to Sonic Youth's "Washing Machine" and being totally blown away once again. Sonic Youth didn't do much great that I've heard after this album, but this album, "Sister", "Daydream Nation" are all amazing albums - and Death Valley '69 might be the single scariest pop song I've ever heard - even more than "They're Coming To Take Me Away Ha Ha", which would give me nightmares if I heard it when I was a kid.
I can't think of any bands with long careers that maintain excellence throughout, but if a band can do a few great albums, that's good enough