Insidious by far, although having said that, I'm not at all a fan on horror films and so I tend to avoid them most of the time. Jump scares and such are not my thing!
Hocus pocus is amazing and brings back great memories of watching it when I was a kid. My absolute favourites are Haloween, Nightmare on Elm street 1-3, Pet semetary, Salems lot....I have loads actually. Been having a horror binge the last few weeks since I've been off work so I've watched most of them already
As far as movies that are actually about Halloween itself, Trick 'r Treat is the main one that I really like. The original Halloween and Halloween III: Season of the Witch are also good. Night of the Demons (1988) isn't amazing but I have a soft spot for it as well.
As for horror movies that I watch near or on Halloween that don't really have anything to do with the holiday itself:
Re-Animator
Dracula
Frankenstein
Bride of Frankenstein
The Wolf Man
The Invisible Man
The Mummy
Night of the Living Dead
Dawn of the Dead (original, 1978 version)
Return of the Living Dead
The Evil Dead (1981), Evil Dead II: Dead by Dawn, Army of Darkness
With all that said, every October I have a month long horror movie marathon and try to find and watch things I hadn't seen before. Halloween is my favorite holiday and I don't really watch movies most of the rest of the year, so it seems like a good way to celebrate.