I assume this thread is about performers who have passed (although there are several very much alive ones mentioned), so here are mine:
I would have loved to have "experienced" Al Jolson at the height of his popularity in one of his shows at the Winter Garden. His ad-libs, energy, and over-the-top persona would have been something to see.
I also would have liked to have seen Zero Mostel, Brando, Gene Kelly, Ray Bolger, and Bert Lahr.
"You pile up enough tomorrows, and you'll find you are left with nothing but a lot of empty yesterdays. I don't know about you, but I'd like to make today worth remembering." --Harold Hill from The Music Man