The Producers is probably his best film, but Young Frankenstein is my favourite.
Blazing Saddles is good, High Anxiety has some funny moments, Silent Movie has some funny moments, History of the World Part 1 is awful. That's all I've seen apart from a small clip from Space Balls, which looked pretty bad.
But I'm eternally grateful to him for getting David Cronenberg's The Fly and David Lynch's The Elephant Man made.