I can't really pick a favorite, so I'll shine a light on some underrated ones:
Lord of Illusions - it's noir mixed with horror. fukking fantastic movie. Scott Bakula killed it. and Famke Janssen?
House of 1000 Corpses/The Devil's Rejects - A lot of folks couldn't get past Rob Zombie as a director, but dude is a horror fan and shows it. HO1C is like your typical "group travels and gets slaughtered" flick, but on mushrooms. The Devil's Rejects is more realistic and even more exploitative.
Wrong Turn - It has your typical "group in the backwoods" setup...except that the victims weren't deliberately stupid, and fought back. Also, Eliza Dushku.
Oh, and don't forget Wrong Turn 2, which was pretty fukkin entertaining too.