This is a fairly old thread, but I felt that I may as well add to it. In my opinion, I'd put TSRTS at number 1, with Earl's court at number 2, though it's really tough. O2 is also pretty good and the solo is nice and consistent, with great vocal delivery from plant. However, the solo is quite reserved, and very short at about 50 seconds, so this brings it to 3rd place. Knebworth is probably the worst of the lot. The solo from the 2nd is hard to listen to. The first night has a lot of nice licks in the solo, but it goes at the bottom because there was some noticeably sloppy playing.
The reason I put TSRTS at number one, is because we're judging the performance as a whole. It has an incredibly (for that year) consistent vocal performance during the outro, which was far better than the vocals for Earl's court. The solo for TSRTS is incredible as well. In the original (unedited) version, he has a couple of errors though (two to be exact), which could potentially bring it down, but the great singing keeps it at number one, and the rest of the solo is excellent, Earl's court had a phenomenal solo. The only issue is that he plays the ending lick an octave lower (or is 2 octaves? I can't exactly remember). There's another mistake during the solo. However, I'd say these two issues are excusable as Page's B string broke. So yeah, great solo, but the outro vocals are really quite bad. Not as bad as the Earl's court night that preceded it, but still not great.
So in terms of these versions, I'd say my ranking is quite fair. Although if you include bootlegged audio performances, you'd obviously get a completely different list.
