I usually like scary movies or comedies above all else. But nothing has made me laugh hysterically since Old School, and nothing has scared me for almost a decade since I watched The Exorcist, all alone at 15, by myself. I made it a memory and shut off all the lights/noise, and watched it alone, and it scared me quite a bit... just thinking about stuff ya dig?
Anyway, bar none, my absolute favorite movie is Valhalla Rising.
A CLOSE second to it would be Goon, and then 30 Days of Night. I don't know why, but I love Vampires, and out of every movie, those seemed the most realistic to me.
