…at whatever you consider to be their peak, who would it be?
Assume good conditions, resurrection, whatever. Top three is fine, if you can’t narrow it to one. There is no wrong answer; tastes differ, after all.
For me, it would be the Who, narrowly edging out the Beatles. While the Beatles came up with some of the most beautiful songs ever, I’d say overall I find the Who better.
Top three?
The Who
The Beatles
…H’m. Creedence Clearwater or the Rolling Stones.
One of Shaun Davey’s immense “festival” pieces, the sort with lots of bagpipers and drummers. One of these is recorded in his album “The Pilgrim.” It’s recorded with a live audience, and you can just hear 'em roar in joy as the pipes open up. I’d love to be in that audience!
I saw all the major artists of the '60s, and a good many minor ones. I was even at Woodstock. But I never saw the Beatles.
And I would love to have been at the premiere of Stravinsky’s Rite of Spring.
And, of course, Florence Foster Jenkins.
Jimi Hendrix Experience.
Velvet Underground.
Nick Drake.
Ozzy with Randy Rhoads.
Jefferson Airplane
Janis Joplin
I’ve seen Jeff Beck.
So, hmm, I’d go with Miles Davis, around Kind of Blue.
Solo Thelonious Monk.
Prince, around 1999, when he’d include How Come You Don’t Call Me Anymore in the set list.
The Stones when they recorded the live Belgian Affair '73 would’ve been amazing, but I don’t think I could handle the crowds.
I’d like to say the Beatles, but when they were at what I consider to be their peak, they weren’t giving public performances, and when performing in the studio, their parts were recorded separately, so it wouldn’t be much like hearing the final versions. Before their creative peak, they were indeed giving public performances, but the amplification technology back then couldn’t overcome the relentless screaming of the fans, so nobody (including the Beatles) could really hear anything.
Queen.
Dean Martin
Jimi Hendrix
Les Paul
I’ve seen many of the heavy hitters from the '60s and '70s, but I never saw Janis Joplin, Jimi Hendrix, or the Grateful Dead. I would have liked to.
I’ve seen just about all I want to see, but I missed Alice Cooper.
I saw Bob Dylan and Joan Baez in the Rolling Thunder Revue, so that more or less fulfills it for me.
But of acts that I never saw live:
The Beatles (though I doubt I would have actually enjoyed seeing them in concert with all the screaming teenyboppers)
Simon and Garfunkel (I’ve seen Paul Simon himself, but never with Garfunkel)
Dan Fogelburg
Absolutely. I got to see them with Adam Lambert two years ago, which was great fun, but to have been able to see Freddie Mercury… 
I have good news for you, you don’t have to say this in the past tense!
I would have loved to catch one of the rat pack’s impromptu late night shows in Vegas.
Van Halen circa 1979. I could only afford one concert that year, and it was between VH and Cheap trick. Not that CT wasn’t good - they were. But I wish I could have gone to both.
Blue Oyster Cult circa Agents of Fortune.
Led Zeppelin. I was going to go to the In Through the Out Door tour, when it got cancelled permanently.
Honorable mention: The Violent Femmes at Century Hall. I saw about half before I fell asleep. What I saw was great, though.
Janis J.
Jimmy H.
Electric Light Orchestra at its most overblown.
I’ll see if I can think of a third. There have been lots of good ones suggested in this thread.
I’ve seen two-out-of-three of those; maybe not in their heydays, but pretty damn good.
Perfection would be in a smokey lounge at the Gold Nugget.