Classic Rock Magazine has a list of the 50 best live acts of all time.
It's a poor showing for me; I've only seen
1. Led Zeppelin
8. Rolling Stones
18. The Clash
19. Thin Lizzy
33. Status Quo
47. Grateful Dead
out of fifty! Of those, I'd rate The Ramones and Status Quo the best live bands I've seen, but it depends on what you mean - I had the best time at their gigs, but I wouldn't listen to a bootleg of either of them.
If you're wondering why Zeppelin doesn't make it, it's because I can't remember a note they played of either Knebworth or Earls Court. Not one single solitary semiquaver. It's possible they blew my mind so hard that I had to blank it out to protect myself, like the victim of an alien abduction, but...
Bands not on the list that I a) can remember and b) were great live - Frank Zappa/Mothers and Hawkwind. Got to give a nod to T.Rex too, but that was in the "not a dry seat in the house" days, so I was more easily impressed.
Worst was Faust. My friend and I walked out after the first chord. To be fair, the first chord had been playing for 35 minutes at that point. I think they may have hated Leeds Uni as much as Leeds Uni hated Faust.