Beyond those few geniuses who have written or composed great musical theater, I agree that there is a wider second category of brilliant men who have shepherded great works to the stage-- directors, choreographers, designers. I'm still miserly giving out the title Genius though:
Jerome Robbins: for changing the way dance BECAME the story in the 1st Golden Age from ON THE TOWN to FIDDLER Hal Prince: for directing nearly a dozen masterworks of the second Golden Age, from SHE LOVES ME to PHANTOM
Of those I'm not sure I'd call a genius-- Bob Fosse: sure he rewrote the way bodies move on Broadway, but his body of great shows is small. Michael Bennet: I'm at a disadvantage, having only seen his SEESAW, ACL, DREAMGIRLS and BALLROOM myself. Can I call him a genius based on 2 masterpieces plus what I've heard FOLLIES staging was like? Sorry, nope. George Abbott: Same disadvantage-- I'm too young to have seen any of his 100's of shows. My sense is he was adverse to breaking traditions though, something a genus tries to do every time up at bat. Tommy Tune: Too few masterpieces to judge.
I have no reservations calling the following designers geniuses, but the word means just a bit less here:
Jo Mielziner Boris Aronson Ming Cho Lee Tony Walton Patricia Zipprodt Florence Klotz Jean Rosenthal Tharon Musser