I think it's just a really tough genre to make a really great movie in. There are so many movies out there that are just average-to-below average because they fall into the same horror tropes and stereotypes and it's a really tough grouping to break out of in that genre.
That being said, when they do get it right, I feel like good horror movies are almost timeless. I really am not that big of a fan of older cinema in general, but I can enjoy a good horror from almost any era, and there are a decent amount of them from almost every decade.
I find I really enjoy the movies that add humor to them and poke fun of the genre (The Evil Dead, Cabin in the Woods, You're Next, Dawn of the Dead, Drag Me to Hell, etc.), but it's tough to really classify those strictly as horror films.
Then there are the technically impressive movies like 28 Days Later or Jacob's Ladder that I didn't find scary, but that I can appreciate as great movies.
As far as (fairly) recent movies that I'd classify as horror and that I found scary, 1408 and Sinister are a couple that come to mind.