I'll have to agree that Christine is a classic. The OP asked us to name films that a horror fan should see at least once, and I'd say one should see Christine.

The Decent, though... I never took to that like a lot of other people did,

i dont care who made it,to me it was just another good s.k book turned into a crappy movie,defiinatly not essential to see,everyone to themselves and all that though

I thought the book was shit and the movie was great haha :D

I loved the book and hated the film..hey!swings and roundabouts innit?

Right on dude. They butchered Christine! The movie is complete shit. The book is an essential read and the movie an essential pass.

yeah,they changed way too much things in the movie that made the story good

My essentials are:

Halloween (original)
Phantasm
Evil Dead
The Exorcist
The Texas Chainsaw Massacre (original)
Suspiria
The Haunting (original)
Night of the Living Dead
Hellraiser

Honorable mentions include:

The Blair Witch Project
The Gate
Return of the Living Dead I & II
The Ring
The Serpent & the Rainbow
John Carpenter's The Thing
Deep Red
The Mist
The Changeling
House of 1000 Corpses
The Exorcism of Emily Rose

Great list. But. I would have to put Exorcist over Phantasm.