It's a tie for most disappointing for me, for completely different reasons.
1a) Sun Kil Moon at... anytime I've seen them, really. Love the albums, and Mark Kozalek definitely has talent, but he is a straight up *******. Just nothing but contempt for the audience and other bands. Just a **** onstage, like he's doing us a favor by gracing us with his presence. And the show just draaaaaags. Way too many of his sad-bastard songs
1b) Ariel Pink's Haunted Graffiti at Bimbo's in SF in 2010. Right after he released "Before Today," which I loved. Label got him this badass touring band of great LA studio musicians to support the album, and he's playing this excellent retro venue in North Beach. This show is gonna be amazing! Or so I thought...
Band comes out, starts playing the opener. And the intro goes on for a minute. Then two minutes. Then three minutes. Ariel is nowhere to be seen. The band is shooting each other glances, like, "Do we just keep playing the intro, or...? This is going on for a LONG time."
Finally Ariel stumbles out from behind a speaker, eating a bag of Doritos. Takes him awhile to find the mic. Clearly wasted on some combination of booze/pills/possibly something intravenous. Starts singing. Forgets the words. Eats more Doritos. Starts screaming. Decides show would be better if he laid down. Just a mess from start to finish. It was the only show I've seen where I felt it was completely justified to whip a bottle at the singer's head.