Weirdly enough, this is a question I think quite often about! So difficult. I'm often between the original casts of South Pacific, Evita,Dreamgirls, Company, or A Chorus Line.
If I had to pick one, I'd probably go with you to see Evita
Interesting a few mentioned shows I saw the original Broadway casts of: EVITA and PARADE. I'm blessed to have been around (starting at the age of 7) to have seen countless now legendary original Broadway casts like: CHICAGO, A CHORUS LINE, ANNIE, A LITTLE NIGHT MUSIC, SWEENEY TODD to even DREAMGIRLS.
I'd have love to have been around to see the original Broadway casts of FUNNY GIRL, MY FAIR LADY, COMPANY, APPLAUSE and FOLLIES.
Probably would have to narrow it down from West Side Story, Chicago, A Chorus Line, Pacific Overtures, Dreamgirls, South Pacific, Hal Prince's Candide and Show Boat... there's way too many to choose from.
Funny Girl with Streisand, early in the run before she grew tired with doing it 8 shows a week and became more like Jerry Lewis than Fanny Brice (so I've heard).... :)
TheSassySam said: "Either "Dude" or "Breakfast at Tiffany's" "
Masochist, obviously.
Since I have to reach back farther to answer the actual question, it would be Ethel Merman in Gypsy.
I'm old, and I saw a lot of these since I started seeing shows in 1969, but my suggestion for the one that would be worth traveling back in time for is the original cast of A Chorus Line. That was life changing.
As for others that have been mentioned:
Applause - Entertaining; otherwise not that great. Bacall was fabulous, but no where near Kathrine Hepburn in Coco that same year.
Company - Yes, it was everything you might have imagined. Inventive staging (that elevator!) sharp-as-a-tack actors, and the thrill of Sondheim finally operating at full power knocking you over. Elaine Stritch must have stared down Dean Jones for two or three minutes of silence - which is an eternity on stage and the audience was with her every second.
Follies - I didn't see it on Broadway, I saw the tour which opened the Shubert Theater in LA with most of the New York cast and it was brilliant. But, I have to say, the London production in 1986 was better.
Chess (Original London Cast) - The London production was head and shoulders above the Broadway version. London, although directed by Trevo Nunn, had Michael Bennet's set concept of the tilting chessboard and the press coverage playing out on banks of large TV screens. The abstract setting served the property much better than the more literal Broadway look. And of course you had Elaine Paige, Tommy Körberg and Murray Head, all from the concept album, playing the roles. Unfortunately by the time Chess hit Broadway the cold war was over, which drained the contemporary feel from the piece.