Damn, seen so many bands it's hard to narrow it down to just a few, but I'll try.
Clutch - I've seen them 9 times going all the way back to '94, and they have never let me down. The only times I've had even the slightest complaint were the times I saw the as a support act and they just didn't play long enough for my liking. Neil Fallon is a captivating frontman and the band never sounds less than perfect. Plus, I've never seen them play even close to the same setlist twice.
Avail - All 8 times I saw them were cathartic experiences. There was never another band like them and never will be. Pure energy onstage. They didn't play perfectly, but it's hardcore, it doesn't have to be perfect. All that matters is that when those 5 guys were on a stage together it was like an explosion. My favorite band ever, live or otherwise.
Iron Maiden - I don't go to huge concerts often, I prefer smaller, more intimate shows, but Maiden is the one band I will pay any price to see up close in a giant venue. Their stage show cannot be beaten. It doesn't even really matter to me what they play, they just always put on a great, energetic show.
Mr. Bungle - Only saw them once, and I'll admit, I was on acid at the time. That said, this was one of the greatest live performances I've ever witnessed. The set was heavy on California, which had just come out a few weeks earlier. I remember being in awe of how tight they were. Also, the show was at an outdoor venue in the middle of monsoon season, so there was all kinds of crazy lightning going on. Just a crazy, crazy show. I'm pretty sure I didn't hallucinate any of it.
Birdflesh - I've never seen a more fun band than Birdflesh. I have two of their albums that I only listen to occasionally, but there's just something about them live. Maybe it's the hilarious song intros, the goofy costumes, or maybe it's just that their kind of grind comes alive onstage. I don't know, they just rule so much. I can't explain it.