The difference between a film you think is underrated and a guilty pleasure is that a guilty pleasure typically has a mostly negative reception, a film you think is underrated could be both that or a film that has a positive reception, but is not that very well known-or remembered.
Some movies I think are underrated:
Happy Birthday to Me, a 1980 slasher
Rock-A-Doodle
The Lion King 2
Cats Don't Dance
Terror Train