I thought the Fiery Furnaces were pretty great live.
I didn't like The Mentors so much on cassette, but they really won me over when I saw them live*.
Actually, this is a pretty tough question. Recently I saw Pale Nimbus open up for The Black Hollies and The Nouvellas open up for The Dirtbombs, and loved both bands. I also saw a few acts at City Gardens in the 80s who went on to become pretty huge in the 90s (Flaming Lips, Fishbone, Rollins Band), but I was usually already converted. In the case of The Flaming Lips, they were opening for The Dayglow Abortions and no one paid attention to them because they were a weird hippie band (I thought they were OK but honestly was not paying much attention to the show).
I think I've told this story here before, but I bought a pink Dayglow Abortions t-shirt with Ron and Nancy Reagan eating a fetus and saying "aargh fuck kill," and my Mom made me get rid of it. I traded it for an Adrenalin OD EP, I think.
Yep, just an old farty-fart ex-hardcore kid over here. I have a theory that ex-punks who don't succumb to drugs or violence wind up in academia.
*not