I have seen every horror film ever. Ever. Here's a few that are decent'ish.
If you haven't seen
Tusk then it's probably best you keep it that way. Those who like the absolutely ridiculously/hilariously fecked up, give it a go. Seriously, you ain't ever seen anything like this.
People going on about The Thing should watch the
50's version. Dated? Of course. It's still a belter if you can appreciate stuff from the dawn of true Sci-fi films.
They Live is a class act. Way ahead of it's time with it's subliminal messaging theme. Full of 80's cheese which is just an added bonus.
Identity Was a bit of an head feck back before every other horror film had a 'twist'.
1408 is well worth a watch. Didn't expect much but definitely enjoyed it.
DO NOT WATCH THIS ...Seriously, don't.
The Cabin in the Woods was a fresh take on the genre. It's like a mixture of Malcolm in the Middle and Friday the 13th. Good fun.
Trick 'r' Treat. It's a bit like Malcolm in the Middle....well, not so much this time, but it's still a bit daft.
Pandorum. A bit like Alien. Maybe. I think it's definitely worth a watch.
The House of The devil. It's got a (deliberate) retro feel to it. Decent, edgy horror though.
Event Horizon. Have people not watched this? It's a proper space horror. Not seen it for years but loved it back then and I'm going to assume it holds up still, and won't let anybody say otherwise. Even if they're right.
The Texas chain Saw Massacre (1971). I'd assume everybody would have seen this, but since Alien and the Thing are new to some people I'd best recommend this. Brutal watching it as an 8 year old.
Noroi: The Curse. Solid Asian Horror. If that's your thing.
Banshee Chapter. Probably a bit shit going off the IMDB rating. I enjoyed it though. Definitely creepy.
V/H/S. Or it's sequals. Several horror subplots well sown together (perhaps) with a binding background plot.
As Above, So below. Solid, claustrophobic horror.
U.F.O Abduction (a.k.a. The Mcpherson tape). All sorts of shite flying around it being a genuine tape and I've not seen it since I was a teen, but I loved it back then.
That's by no means a comprehensive 'best of' list. Just a few worth watching (or, in the case of one or two, not worth watching IF you are a normal person).