[Van Hagar]

Acts I regret not seeing (I'm only counting from mid-70s on when I could have, so no Beatles or Woodstock).
AC/DC with Bon Scott.
Skynyrd with Ronnie Van Zant
Queen with Freddie
Rolling Stones when they were younger

Acts I'm very glad I got to see once
Prince
ZZ TOP before MTV sellout
Van Halen when they were young and the world's greatest party band
Tina Turner on her farewell tour (one of the few acts who did a farewell tour then actually stayed retired.)
Dixie Chicks before Nashville blackballed them

There's a bunch of my absolute favorites, like AC/DC, Travis Tritt, Guns and Roses, etc. that I've seen multiple times, just listing here acts I only saw once.