I wouldn't call "Charade" under-rated. TTBOMK, this film is very well-thought of. A lot of people you may talk to today may not have seen it, but after all, it is forty years old.
I think "Clue" was and is under-rated. A lively slp-stick farce with a skilled ensemble cast and 3 different endings.
What for me may be the greatest mis-rated film is "Ishtar" with Dustin Hoffman and Warren Beatty. Panned by many (most) as perhaps the worst major motion picture ever made, I found it to be, as intended, reminiscent of the Hope-Crosby road films and full of funny bits, which is after all, what comedy is about - being funny.