Pretty good list and most I actually have had in my garage on VHS because of no DVD. It is really hard to judge underrated because a movie like Paperhouse got 4 stars from Ebert (and The Body Snatchers & Island of Lost Souls get consistent high reviews), yet remains obscure and OOP in the US. That said, here are some of my fave underwatched horrors that don't get discussed here too much:
House of Dreams (1963)
Blood & Lace
Bloody New Year
Nature of the Beast (or Clownhouse)
Urban Legends: The Final Cut
Dark Waters (the 40's noir meets southern gothic.)
Scream for Help (hilarious)
Footprints (maybe it has a cult now)
Future Shock (anthology-love parts 2 & 3)
Track of the Vampire/Bloodbath
Ginger Snaps 2 & Creature Walks Among Us (overshadowed by the first)
Return in Red
and a bunch mentioned in Nightmare USA.
I will be adding to this list as they pop up in my head.