Anything, and everything, by Almodovar should be on any film lover's list (Those that see beyond the mainstream. It's not a fault, just makes no sense).
Any japanese horror seems to hit the mark for me, they just do it so well, even a contemporary concept like Battle Royale has been attempted and failed in Hollywood. The Eye, The Grudge, Ring, etc, are all fantastic aslong as you get the originals.
Irreverible is stunning, as are all of Gaspar Noe's films, if you can take them. Easy watching, theyare not.
Happiness of the Katakaturis (sp?) was brillaint when I first saw it, and i'm always menaing to pick it up from HMV, but I always see something else.
The Isle has moments of brilliance, and the film is very good on the whole, as is Dark Water.
I'll just start listing now, or i'll go into film ramble mode:
Gozu
Old Boy (Caution: Lovers of Octopus' beware)
Bad Education
Bon Voyage
Akira
Urotsukidoji (For when your back from the pub, hammered, with a couple of joints)
Suspiria
The Bird with the Crystal Plummage
Sex and Lucia
Sweet Sixteen (subtitles for the non-scottish for the furst part. Genius!

)
List really is endless. Just get Shaun of the Dead and laugh yourself sick.
