Stevie Ray Vaughan, of COURSE – and specifically his last show, at Alpine Valley. My Texas citizenship requires me to answer thus, although I would have anyway.
As for the other two, I immediately thought of these two before I clicked on the thread – and how is it that nobody has mentioned them?
[ul][li]Oingo Boingo, and[/li]The Tubes.[/ul]
Johnny Cash
Jim Croce
Buddy Holly
Bob Dylan and the Band (then still the Hawks) in 1966.
The Who with Keith Moon.
The Clash.
Well, three just are not enough, so here’s more I can think of:
Elvis Costello and the Attractions in 1978.
Otis Redding.
The Flying Burrito Brothers with Gram Parsons and/or Gram Parsons and the Fallen Angels (with Emmylou Harris).
The Beatles in Hamburg or at the Cavern Club.
Joy Division
The Smiths
Lush
The premiere of Beethoven’s Ninth.
“*Rachmaninoff! *It shakes me! It quakes me! It makes me feel goose-pimply all over! I don’t know who I am–or where I am–or what I’m doing! Don’t stop–don’t stop–don’t ever stop!”
(Sorry–having a Marilyn Moment)
I’d go with my childhood heroes:
KISS, circa '77 or so. I was a serious devotee and a member in good standing of the KISS Army.
Van Halen, same circa – maybe a few years later. I saw them with Sammy Hagar in the 80s and I saw David Lee Roth on his solo tour, but I would have loved to see the original line-up when they first hit their stride.
And to round out, I’ll go a little later and say, Stevie Ray Vaughn, at any time in his career.
Now I want Champagne and potato chips.
Charlie Parker
Vladimir Horowitz
The premiere of Stravinsky’s “The Rite of Spring” in 1913 which (may have) caused a riot
of the Parisians as it was far above what their primitive minds could understand.
The Beatles
Jimi Hendrix
The Replacements (on a good night, with Bob Stinson)
All these people saying Stevie Ray Vaughan makes me realize how lucky I was to see him perform a bunch of times, including his last show as a headliner on Aug. 24, 1990- at the Kalamazoo County Fair! I still have the ticket stub, the price was $10.
I used to say “Man, I wish all those 80’s hair metal bands I used to go see in the clubs were still around…I’d kill to see them again”.
Hey, whaddayaknow, the nostalgia wave has hit that era, and I’ve been seeing a ton of them again!
Black Sabbath 1975 - of course I assume everyone knows Black Sabbath continued for 30 years after Ozzy left, but they were never the same. Saw them once, in the '80s, with a very forgettable lineup. Apparently there is a reunion in the works.
Motorhead 1980 - with Fast Eddie Clarke. Motorhead has been up and down, hit and miss since his departure in the early '80s. Another groundbreaking band/sound. Saw them late '80s, was a great show.
Metallica 1985 - with Cliff Burton. Burton’s death in '86 in a tour bus accident assumedly drastically altered the ultimate direction the band took. He was the most accomplished musician of a talented group. I saw them a few times late '80s early 1990, before the dreaded Black Album.
Well, let me scratch that itch, baby.
Janis (big shockeroo there, eh) with Big Brother, natch.
John Denver
Jim Croce
Benny Goodman’s Carnegie Hall Concert in 1938
The Tommy Dorsey Orchestra, when Bunny Berigan was his trumpet player, on a night when they performed “Marie”. (Berrigan’s solo begins at 1:43, and will make your jaw drop!)
King Oliver and his Creole Jazz Band, circa 1923, when Louis Armstrong was playing second cornet.
Labdad, I wish to be your date. I’ll buy the popcorn.
ETA: And Spike Jones.
Simon and Garfunkel (pretty much whenever)
Alan Parsons Live Project (I know they’re still touring, but I’d go back to the days of Alan, Ian Bairnson, and Stuart Elliott (and if I could get Andrew Powell in there too, that would be great))
Peter Gabriel’s “Up” tour (his live performance of “Signal to Noise” was nothing short of orgasmic)
Bonus: Men at Work, Business as Usual tour (this was the first concert I ever saw, and it was great)
Bonus 2: Styx’s “Kilroy Was Here” tour. I wanted so much to see this tour when it was live, but I was in high school and my parents wouldn’t let me to go L.A.
Bonus 3: Tom Lehrer 
Mrs. Cad and I had this conversation a while back. Needless to say, everyone in their prime.
*If you want to point out Moreschi, the recordings are poor and he was well past his prime when he recorded even if his ability were not questioned. Plus his style was very different than the 18th Century opera that were the rage during the height of castrati singing.
Elvis - In Vegas just to experience old-time Vegas and the show but before his voice really left him.
Take your pick: The Beatles or U2 back when they were just starting out in the little clubs.
Honorable mentions) I would have said the premier of Tosca but Toscanini was not available to perform and Enrico Caruso was not in the cast. Imagine llistening to him singing E lucevan la stelle. La boheme? I saw Baz Luhrman’s version of it.
I took Mrs. Cad to see one of her favorite performers that she had never seen live - John Denver at the Cerritos Performing Center. It turned out to be his final show.
Great choices, I’ve seen almost everyone at some point but as far as classic shows:
Alice Cooper- in Detroit in 1969 and the 1973 shows
KISS - 1976
The Ramones -1975
Van Halen - 1978