Halloween III: Season of the Witch - Most people automatically hate this movie because Michael Myers is nowhere to be seen (well, actually he is... on a TV in a bar) but aside from this, the movie is creepy as fuck and good luck getting the "*insert number of days here* to Halloween" Silver Shamrock song out of your head after watching it. I like Tom Atkins in it and Dan O'Herlihy is kind of a douche as the CEO of Silver Shamrock. I'm a fan of horror movies, but this movie has one of THE creepiest and disturbing deaths out of all the movies I've seen (where the mask is tested out on the kid).
Saw I - VII - A lot of people dismiss these movies as torture porn, and while they can be a bit gruesome and the acting a bit wooden, I still like them all the same. All the movies build on the events of the others and story threads are carried over through multiple films, and the end of Saw 3D is pretty badass. For movies that were cranked out one per year, the traps are pretty inventive and the plot twists pretty shocking.