To echo what others have said, Hairspray was definitely his best choreography.
As for shows he's directed, I think the original Broadway production of Legally Blonde was weighed down by having too much scenically--every location did not need to be so fully realized. The set kind of dwarfed the story. The London production was a more reasonable size (though, still, by no means modest) and allowed room for the characters to breathe. Plus, the edits to the book were small changes that made big impacts. So, by the time it got to London, I'll say Legally Blonde was his best directorial work, but Kinky Boots is definitely very skillfully done.