Best horror movies to watch?

A few that haven't been suggested yet:

Suspiria
Inside / A L'interieur (Sometimes easier to find by its French name. Also, don't look this one up on Rotten Tomatoes because some bright spark decided that revealing a massive spoiler worked as an adequate review summary. You really have to be an absolute douchebag of a critic to include that in a review in the first place.)
The Silence of the Lambs (Borderline horror / thriller.)

Personally, I'd suggest sticking to old horror movies because even if they don't have all the special effects and what not I find them to be more entertaining and creepy than most of today's horror movies.

The Silence of the Lambs (1991)
The Exorcist (1973)
The Shining (1980)
Psycho (1960)
The Thing (1982)
The Evil Dead (1981)
The Omen (1976)
Alien (1979)
A Nightmare on Elm Street (1984)
Poltergeist (1982)
The Fly (1986)
Hellraiser (1987)
Carrie (1976)

Newer movies I enjoyed

The Descent 1 + 2
Babadook
Pan's Labyrinth
Drag Me to Hell
Orphan
The Orphanage
REC 1
V/H/S 1, 2 + 3
Pulse 1, 2 + 3 (Even though I prefer the Japanese version of it a lot more.)

I think there's more but those are the ones I can think of right now. There's also really good and creepy Japanese horror movies and I'd always suggest the Japanese originals over the American remakes anytime.

If you want a more recent film that will geniunely send chills down your spine and leave you feeling disturbed.

I would suggest The Babadook

You will not be comfortable.

It's my only counter-point to the argument "modern horror is dead"