Gentlman Prefer Blondes because it was Marilyn at her absolute best! She is such a fantastic actress and this film captured everything that was great about her. (2nd would be 'The Misfits'. It was her best dramatic performance but its kind of hard to watch because it was her last completed film.)
Marilyn was extremely beautiful in Niagra but it was a rather scary unnerving movie, albiet very scenic. I liked her breakout performance in 'Clash por Night' with Barbara Stanwyck. Barbara is a wonderful actress. Come over to the Barbara site and be fan # 50. She deserves a multitude of fans! Marilyn was awesome in 'The Seven año Itch'...Rachmaninoff lol so amazing! Oh, and of course 'Gentlemen Prefer Blondes'. The song scene is incomparable - the best in cinema history in a lot of ways.