Ooh. Good one.
There are a lot of fine candidates, and “Echoes” is one of the rare songs from their setlists of the early 70s that tended to vary only in small ways, whereas most of the other non-Dark Side material they played around that time could change shape and length radically from night to night.
I’ve always like the split-in-two Pompeii version, actually—the first half in particular is very intense. It’s also split in half, though.
A few full versions that come to mind include the May 19th, 1973 version at Earl’s Court (a generally incredible show), the Nov 16th, 1974 version at Empire Pool, and the September 22nd, 1972 version at the Hollywood Bowl.
This last ranks among the best shows they ever played, and this take on “Echoes” is amazing—the band was on fire when it hit southern California in 1972. The intro is super-spacey, with a prominent slide guitar lead, and Wright’s organ work is really interesting. Gilmour’s fuzzed-out lead after verse two gives me chills. Also, the middle “scream” section is really out-there, with a lot of dissonant organ.
The Earl’s Court version is one of the heaviest they did—the double stops after the final verse hit very hard.
A dark horse candidate might be the version from June 28th, 1975 at Ivor Wynne Stadium in Hamilton, Ontario. Gilmour does some really odd things on the intro, and they come in at an unusually slow tempo that gives it an even more ethereal effect. This is also one of the late versions that includes a sax solo after the second verse, which never quite equals Gilmour’s usual work on that section, but lends an interesting feel nonetheless. I have a fantastic crowd source of this show—I’ll try to find or put a link up later this week.
If you forced me to pick, I’d probably go with Hollywood Bowl.