Close seconds would be West Side Story for the score but not the book and A Little Night Music for the book and the score together.
But the all-around writing honors I would, with a little ambivalence, give to Next to Normal.
It takes contemporary lives and problems, both personal and societal, and tells a story with an imaginative use of music and movement and opportunities of everyone in the cats to shine.
It's a silly question, actually, but I would go with Next to Normal, and I think that, were they alive, Lenny and Jerry might agree. (Not Athur, of course.)