I saw the 2013 production of Einstein and it was absolutely fabulous. They did a very nice taping of it that you can watch on Youtube that, while obviously not being the same as seeing it in real life, is extremely well done.
Anyways:
-Roar of the Greasepaint, which I don't think has ever had a revival. I've never been able to read the book for it, but I love the concept, and the score is, obviously, great. I'd love to see this come back, but it's hard to imagine topping Newley and Ritchard
-The Life And Times Of Joseph Stalin, which wasn't Broadway but is something I would give one of my kidneys to have seen (see also: United States Live and the full 24 hours of Taylor Mac's History of Popular Music)
-1600 Pennsylvania Ave. I've listened to the soundboard recording endlessly, and from what I've heard the actual production was fairly barebones (evidence supported by the scant photos I've been able to find), but it would be a huge thrill to see and hear it live
OBC Follies, Chicago, and Evita all get runner-up, as well as the original production of Hellzapoppin, early-previews Merrily, and the Streep/Julia Taming of the Shrew, which is so tantalizingly glimpsed in Kiss Me, Petruchio.
Oh, and the '74 Candide. And the OBC of Nine! And...and...