Labeling a band as the greatest band of all time is kinda hard. There are so many greats. Plus, with the oh so many different genres (hard, soft, classic, alt, metal [which has it's own group of genres too], indie, pop-rock, nu, grunge, etc.) within rock, it's hard to compare them all.
I'm not a big Beattles fan, but they've no doubt shaped the music world. They've had great songs, and songs that you just can't love so much. I can't really label them as the best ever.
Zepplin has no doubt had so much talent in the band, but I'm unfamiliar with a lot of their music.
I'm a big Grunge and Alternative fan, so... I can't really talk about too many bands that popped out before the 90's. But, after thinking it through, I will through out a name for one of the greatest bands ever.
Nirvana.
They aren't rock. They aren't a band. More like a techno/electronic group/duo.