Children of Men
I rented it the other day, haven't watched it yet, probably will today.
Death To Smoochy
Big Fish
The Princess Bride
Donnie Darko (I guess this one has a bit of a cult following, but I'd still call it underrated)
Fido
Phantasm
FIDO was great! It was vastly overlooked and a fun and interesting film. Carrie-Ann Moss was really something in that film, too.
Hubba Hubba
There are so many "foreign" movies that are awesome that get totally overlooked by western audiences...