I think the first set that really impressed me was Oliver Smith's set for Hello Dolly! The show was a wonderful musical, to begin with, but that set moved, danced, folded, unfolded; it was truly amazing.
Smith's work was always amazing: I saw the original productions of The Sound of Music, Camelot, and Hello, Dolly, all designed by Smith. The Sound of Music didn't impress me, Camelot was really gorgeous, but Hello Dolly was the first show I saw with a set that was more than painted drops.
Boris Aronson's sets for Follies were also amazing. If anyone ever needs a designer for this show, the first question for any designer in interview for the job should be how to get from a ruined theater to Loveland and back again.