Really, really hard for me to choose, but I went with Call of Cthulhu.
There's nothing else quite like it and it is a type of horror that I find very fascinating.
The other very close contender for me is All Flesh Must Be Eaten. I ran a really awesome one-shot adventure that cemented in my mind just how cool that game is. Only one shot was ever fired (a hunting rifle, the only weapon in the party, at point blank range) and it missed.
My brother also ran a fantastically moody, atmospheric and involved game where the players were entirely absorbed into it and role played superbly.
Gawd, I love that game. I've got to run it again soon...
HARRY DRESDEN — WIZARD
Lost items found. Paranormal Investigations.
Consulting. Advice. Reasonable Rates.
No Love Potions, Endless Purses, or Other Entertainment.