Zep with John Bonham.
Queen with Freddie.
Big Brother and the Holding Company with Janis.
Gasp!! This must be corrected! So do it. Do eeeet now!!! 
Okay, so he tops my three, only I wish it was '72 - '73 during the Billion Dollar Babies tour. Which I’m sure is what I say every time this topic comes up. He is THE man.
#2 is Pink now because I adore her and she kicks ass, and #3 would’ve been Keith Green before he died in a plane crash. When I was a teenager and heavily into Christian music, I was devastated to find out he was gone after loving several of his songs. He seemed so genuine and sincere in his spiritual beliefs (and that felt like a much more innocent era), I still would be thrilled to have seen him in his heyday.
I would kill to go back in time and see the Brubeck Quartet at Carnegie Hall.
Miles Davis when he had his nonet with Gerry Mulligan, Max Roach, et al. Or the sextet with Coltrane and Cannonball Adderley.
Janis Joplin for the energy blast.
Even though the Beatles aren’t even close to being the best group ever, I would’ve loved to be on that rooftop in 1969.
Hendrix
Steppenwolf
First, without a doubt, Aretha Franklin. In any time of her career. Sigh, it’ll never happen, she won’t tour Europe again in her lifetime.
Second, the Beatles 1962, either at Star Club, Hamburg or at the Cavern.
Third, Van Morrison with the Caledonia Soul Orchestra on his 1973 tour, which is documented on his 1974 live album It’s Too Late To Stop Now (and lately completed with the fantastic 3-CD set It’s Too Late to Stop Now…Volumes II, III & IV) That’s Van at his peak with a dream band including brass and string sections, pure magic. Alternatively his 1980 performance at the Montreux Jazz Festival, which was almost as transcendental. I’ve seen him live in 1992 in Cologne and though it was a solid concert, the former magic was somehow gone. He was always and foremost, since the age of 15, a working musician, but he, well, transcended this on these former concerts, whereas that gig at Cologne and footage of later concerts I’ve seen or heard since then (for instance the 1994 live album A Night In San Francisco) felt always like going through the motions.
And hundreds more, but we were restricted to three :).
Hamilton, original cast.
Simon and Garfunkel.
Ookla the Mok
Bolded mine.
My answer word for word.
I would have loved to attend theLast Waltz in 1976. It could have happened.
Aaargh, and this, first leg of the Rolling Thunder Revue or even better, Bob Dylan and the Hawks 1966 in England, preferably Manchester Free Trade Hall.
(Well I’ve seen Dylan in 2000 in Oberhausen, but that was a different thing, a different voice and a different time. But it was good).
I’ve seen Yes five or six times, but as I was born the year Fragile was released, it was their later incarnations I saw. I would love to see a show with the classic Fragile lineup, or even a '70s show -Bruford +White.
I did see ABWH in '89, and the Union tour in '91. At one point during the latter show, the Fragile lineup (only) played “Long Distance Runaround.” That’s the closest I’ll ever get. 
And now that Yes is basically a cover band led by Steve Howe, I won’t be seeing them again.
David Bowie, Diamond Dogs Tour (1974). Coked to the gills, but IMO at his peak as a live performer. And I’ve seen him in concert four times, each one was great, but Diamond Dogs show was (from what I have heard) amazing.
Queen with Freddie. Any time.
On a completely different type of music - Fairport Convention with Sandy Denny. Or maybe Ella Fitzgerald in her prime. I don’t know.
Beatles
Sex Pistols
Ziggy Stardust '72
Again, assume perfect conditions*, and the set list can be anything from their repertoire.
So, no screaming teenies at the Beatles, and they’d be performing anything that they worked on together.
Perfect conditions would be a large crowd for most groups - well behaved, but into it. Others may be a small coffee house setting.
The 1/30/1969 rooftop concert. It was short, but of any Beatles concert that’s the one I’d want to be at.
Ooooh, a good runner up might’ve been the Doors concert where Morrison decided to let the Lizard King out.
Bowie Ziggy Stardust Tour '72
Bowie Diamond Dogs Tour '74
Roxy Music any show of first two tours with Brian Eno
Pink Floyd The Wall tour '80 since I could’ve went but like an idiot, didn’t
Kesha (already seen her four times, and hopefully many more in the future)
Tom Petty (ditto, except two instead of four previous times)
Michael Jackson (never saw him and never will)
Bix Beiderbecke and his Wolverines in the 1920s. Bix died at 29, but they say his cornet sounded like a girl saying “yes.”
I’ll share a table with Chefguy at Birdland for one of the only two live performances of the 1948 Miles Davis “Birth of the Cool” nonet with Gerry Mulligan, JJ Johnson, Lee Konitz, Bill Barber, John Lewis, etc.
The 1905 premiere of Richard Strauss’s Salome in Dresden.
Genesis. Probably on the Abacab tour.
Marillion. After Clutching at Straws, before Fish left the band.
Pink Floyd
Joy Division
The Smiths
Shelleyan Orphan
None of mine are current but damn, there’s nothing but a bunch of old fogeys in here.