I've read Invisible Cities.
I'd back Rabbit Run when the 1950s comes around. It's a great book. I've been meaning to read another Updike, but I won't nominate it here.
I'd like to nominate One Flew over the Cuckoo's Nest, but it appears that many have read it so I'll save that for my own reading.
I'm going to nominate
The Bell Jar by Sylvia Plath.