I've easily seen... well, a lot of fucking horror movies.
To date I would say 1% are great must watch films, 10% are good and the remaining 39% are OK and the final 50% are utter garbage.
I keep all the really good horror movies and outside of the classics (last 10-20 years) there have only been a handful of movies that stuck out to me.
Sinister
Evil Dead
Some of the other notables in my collection at John Carpenter's The Thing, The Exorcist, Child's Play (I was very surprised how well this held up, and how humorous it was), The Shining, Alien/s. Then it goes into just good films like a first viewing of Saw, The Blair Witch Project, Cabin in the Woods, The Conjuring, The Ring and Insidious.
I've dabbled with posting reviews somewhere but I really can't be fucked to care enough.
One caveat to note is that horror films based off rape/torture shock scenes and nothing more (like The Hills Have Eyes, for example) rank extremely poorly for me. You raped 'em. Good job. Where's the fucking content? Asshole.