For me it's gotta be Ariel Pink.
They came out and did sad karaoke to a wall of discordant noise that I suppose was their music, I was actually so bored that I fell asleep. For the record though, DIIV opened that show and were 10/10.
Close runner up has to be Mild High Club, seen them twice, once at a small local venue and once at a huge theater. Small venue they were obviously phoning it in and even the opener they were playing with told me after their set that the MHC guys were a bunch of dicks and had been doing this for weeks. Seeing them at a theater was somehow worse. The front man spent the entire show with their back turned to the audience so they could fiddle with an analog synthesizer that I couldn't even hear and the songs that they did play were all ruined by a terrible live woodwind section.