The original Sunday in the Park with George had spectacular design.
That show had a lovely imaginative and humorous set. Ones I've really liked:
Oliver Smith: Camelot, visually stunning; is the art of the painted drop a thing of the past?
Oliver Smith: Hello, Dolly!
Raoul Péne duBois: No No Nanette: three stunning 1950s settings
Boris Aronson: Follies; no one's gotten close to it since then
Boris Aronson: A Little Night Music
Tony Walton: Pippin, an amazing mix of commedia dell'arte, macrame, and Romanesque art