Working shift work for the past 20 years, there are so many. But in my younger days I would have to say it'd be:
The Germs - by time I got into music they had stopped playing, but there was the Darby Crash band and then they did that re-union/final show in December of '80 that, being 14, I couldn't manage to get a ride to.
The Screamers - they were still playing a bit towards the end of '80 as I recall, but I was "too punk" to get it. Outside of a few demos Rodney Bingenheimer would play, they were more known to me for that Gary Panter logo that seemed to be so prolific.
The Clash - There were several opportunities to see them, yet again, I was "too punk" (aka too young and dumb) and they were too comercial. I remember using my limited funds to go see the Angelic Upstarts instead during an '82 7 night stand by the Clash at the Hollywood Palladium. The upstarts were playing just a stone's throw away from the Palladium. By time I realised the error of my ways I saw the "new" Clash on their 'Cut The Crap' tour. Not the same thing obviously.
The Jam - What can I say, my "mod" friiends were smarter than me. I passed and now regret it.
Bags - Another band I never caught thatI could of towards their end.
Ray Campi - How and why I haven't seen him I do not know! There's still a chance to though!
Mac Curtis - see above
The list is actually quite long now that I start thinking of it, so I leave at that.