Of the shows Armani mentioned, I'd say Royal Albert Hall 1970 (they played there in '69 but there are no known tapes), Earls Court 1975, and L.A. June 1977 are the most highly regarded. Seattle '77 is very long and interesting in some respects, but far from one of their best shows. Parts of it are a train wreck.
I also really like the July '73 NYC "The Song Remains the Same" shows - but again, not their best.
In addition to these, I agree with the previous commenters that the Europe '73 tour was among their strongest. Southampton sounds great but Plant's voice is a mess, and the performance is a little uneven (although the HMMT-Communication Breakdown encore - oh my!). The UK and continental shows in March, however, are probably the strongest. Offenburg (as Wolfman says) is considered the best, but I think Essen and Vienna are pretty terrific too.
And then there's Japan 1971 - legendary, and deservedly so.
As for the single best performance, that's nearly impossible to answer, because the band sounded so different, and played such different sets, in different eras.
It's somewhat arbitrary, but I would probably rule out shows that didn't feature a full, long set (so that rules out almost everything before 1970), and that didn't include any acoustic numbers (which rules out the 1975 U.S. tour). I'd also rule out anything in '79 or '80 because I can think of only one performance (07 24 79 Copenhagen) that, as a whole, could rival their best performances in any other era.
So for a Top Five performances, I'd say you could start narrowing things down as follows:
(1) The 09-04-1970 LA "Blueberry Hill" show
(2) The 06 25 or 27 LA/Long Beach "How the West Was Won" shows
(3) Your favorite European date from March '73
(4) Your favorite Earls Court show (IMHO either May 24 or 25)
(5) Your favorite June '77 L.A. Show (Most folks seem to think 06 21, 22 or 23 is the best)