Best Live Album Ever...

A lot of good ones mentioned. I'll add Poco's Deliverin'.

Robin Trower "Live"
Gentle Giant "Playing the Fool"
ditto on Little Feat "Columbus"
ditto Santana "Lotus"
where-as I love YES, generally, "YESSONGS" has to be one of the worst sounding live albums of all time... sounds like a 4-track cassette with bad tape and lots of limiting. Having said that, Steve Howe's performance on "Yours is no Disgrace" and the electric version of "Roundabout" are rip-roaring make-me-feel-18-again performances.
Hey how about Bob Marley's "Babylon by Bus" UK pressing with BIG bust a gut bass.

Emmylou Harris - At The Ryman

Dead Can Dance - Toward The Within is quite good.

I just counted and I own 491 Live albums on CD - that means the success rate is pretty low given the following list of exemplars.....

Renaissance - Live at Carnegie Hall - easily my favourite live album. Fantastic performance and audio (especially the latest Eroc mastered one)

Other notables:
Be Bop Deluxe - Live! In the Air Age
Deep Purple - Concerto for Group and Orchestra
Deep Purple - Made in Japan
Flower Kings - Alive on Planet Earth
Dan Fogelberg - Greetings from the West
Frumpy - Live (Crap sound, great performance)
Grand Funk - Live (the most exciting live album I've heard)
Hendrix - Band of Gypsys
Jethro Tull - Bursting Out
Jethro Tull - A Little Light Music
Jethro Tull - Carnegie Hall (from the 25th anniv' box set)
Loreena McKennitt - (any and all LM live albums are excellent)
Sarah McLachlan - Mirrorball
Odin - SWF Session (stunning album)
Pink Floyd - Pompeii (only on DVD or bootleg CD)
Pink Floyd - Pulse
Pink Floyd - KQED (boot)
Pink Floyd - DSOM Experience Edition
Porcupine Tree - Coma Divine (SE)
Renaissance - At the Royal Albert Hall (aka King Biscuit Hour)
Richard Shindell - Live at the Chandler Music Hall
Bruce Springsteen - Live at the Hammersmith Odeon, 1975 (from BTR 30th box)
Stream of Passion - Live in the Real World
Traffic - Welcome to the Canteen
Wicked Minds - Live at Burg Herzberg
Wishbone Ash - Live in Chicago