I think 48 Hours is still his best movie, closely followed by Beverley Hills Cop, and then Metro. Metro had some silly moments, but I think it could have been a very good movie, if not for some lame attempts to turn it into a comedy.
And it's probably the closest Murphy has come to doing a serious role (though I have not seen Dreamgirls).
And to that end, I have never understood why Murphy has never done any serious acting roles. Most comedy actors have at least given it a go, even if not always successfully.
I think he has it in him to do it, but I get the impression that he has either decided, or been badly advised to stick to what he is best at...only the law of diminishing returns came into play.
Basically, people got fed up of his schtick. He made some attempts at 'sophisticated' comedy in films like Boomerang and Distinguished Gentleman, but after Nutty Professor it seems that he has been content to plough the gross-out/kiddie market type of comedies.
He is fast becoming the Steven Seagal of bad comedies...and will soon end up in Direct-to-DVD hell.