Really tough question, so I'll ignore it and answer with a few...Lee J. Cobb in Death of a Salesman, Len Cariou in Sweeney Todd, Ethel Merman in Gypsy and On The Twentieth Century, my favorite show album.
All the original productions of the Prince/Sondheim shows from the 70s and anything with Gwen Verdon
I think it would have been cool to see Streisand in Funny Girl and Ray Bolger in Where's Charley? and I often think about how crazy and exciting it must have been to see Oklahoma! without knowing anything about it, so my answer is original cast, Follies.
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