Led Zep all the way. They had it all- legendary singer, excellent drummer and bassist, and of course, Jimmy Page. 4 symbols consistently gets voted as one of the best rock albums ever, Stairway as one of the best rock songs, and Page as one of the best guitarists. They will never stop being influential to rock bands that came and go and I still find their music exciting. To me, they are one of the only bands whose music gets better every time I listen to it.

I have to confess that I'm also a bit surprised at the weight of support that G'N'R are getting on this thread. To me they only ever released 1 really good album (appetite), 2 very mediocre albums with some enjoyable highlights (use your illusion 1,2- which I think only had enough good songs for one really good album- but instead they seemed to flush quality control down the toilet, went for the money, and released 2 average albums with some very poor material which they obviously hadn't spent much time developing, and which also had some really good stuff), and the other albums aren't worth mentioning. Also, as much as I respect Slash's incredible playing, he isn't very original- I find his playing a bit derivative of Jimmy Page's playing (also of Angus Young and Jeff Beck's playing). For me, they just don't have what it takes to be up there with the greats- Led Zep, deep purple, black sabbath, AC/DC, Queen, rolling stones, jimi Hendrix, the who, pink floyd, etc.. never mind be the greatest.
But that's just what I think...