I've not watched many new horror type movies in the past 5 years or so and haven't seen too many "classics," so my list is somewhat limited. But, I'll give it a go.

I honestly can't rank them as that would take a lot more thought than I can afford to devote to it, so these are in no particular order.

I'm not sure if these all fit as "Halloween," but I like them.

Army Of Darkness (1992) - Hilarious.
Night Of The Living Dead (1968)
The Crow (1994) - One of my all time favorite movies.
The Exorcist (1973)
Mothman Prophecies (2002)
The Monster Squad (1987) - Watched it yearly when we were younger, now I can't not watch it.
(Tales From the Crypt:) Demon Night (1995)
The Prophecy (1995)
The Thing (1982)
The Shining (1980)

Since this is "no particular order," some mentionables:

Se7en (1995)
Constantine (2005)
The Good Son (1993) - Culkin played an awesome creepy and psychotic little kid.
Bad Moon (1996) - I don't know, I thought it was a cool werewolf flick.
The Ninth Gate (2000)
Saw (2004)
Texas Chainsaw Massacre (1974)
Hellraiser (1987) - I love Pinhead and the Cenobites, not to mention an evil Rubik's Cube.
Evil Dead II (1987) - These should've made Campbell a horror star.
Wolf (1994) - Awsome werewolf movie.

EDIT:
Ghostbusters (1984)
The Nightmare Before Christmas (1993)

Most of the films previously mentioned I'd watch in a heartbeat. I thought I'd pick 10 movies that haven't been brought up yet. 

1. Freaks (1932 classic about sideshow freaks - simply incredible)
2. The Seventh Sign (1988 film about the Book of Revelations)
3. The Reaping (2007 film about the 10 Biblical Plagues - I sense a theme here)
4. Serpent and the Rainbow (1988 zombie flick - this one will stick to you long after the credits have rolled)
5. The Hunger (1983 vampire movie - a must see for any vampire movie fan)
6. Hideaway (1995 film based on the novel by Dean Koontz)
7. Wolfen (1981 werewolf film - awesome flick)
8. The People Under the Stairs (1991 Wes Craven film - the title says it all)
9. The Changeling (1980 ghost flick- still creeps me out)
10. It's the Great Pumpkin, Charlie Brown (Oh you know you like it, someone had to mention it ha ha)

*runs off doing a Snoopy dance* 

Definitely this one, too. Great movie.

I forgot to mention The Serpent And The Rainbow.  What a great movie.  I haven't seen that one in awhile.  I love it.

This is a tough one...

1) Halloween (John Carpenter's and Rob Zombie's)
2) Texas Chainsaw Massacre (original - Gunnar Hansen (leatherface) and Edwin Neal (hithiker) are great guys
3) Friday 13th movies
4)Nightmare on Elm Street Movies
4) The Exorcist (original) (I saw it as a kid at the drive-in and still gives   me the willies)
5) Freddie vs. Jason (Great villain vs. villain) (Would've been cool if Pinhead snatched them both to hell at the end)
6) Hellraiser I and II
7) 13 Ghosts
Dawn of the Dead (The new One)
9) Land of the Dead (George Romero)
10) Resident Evil films

Scorpius