I'm going with the 2010s followed by the 1970s. The 2010s are severely underrated because they're so close to current times. The sheer volume of fantastic horror films from the last decade, is unrivaled at any other decade. I think it's going to take years for enthusiasts to unpackage it all and have a semi-accurate view of how they all rank and hold up to films of other decades.
The 80s were good up until The Thing in 1982 because it was basically the end of 1970s horror films. But then came a wave shitty slasher flicks that were more or less horror-slapstick comedy.
But basically, my favourite horror films of all time are as follows (In reverse chronological order):
* Conjuring 2
* Conjuring 1
* The Thing
* The Shining
* Dawn of the Dead
* Invasion of the Body Snatchers (78)
But there are A TON of other films I like. The only slasher films I think are good were Candyman, Halloween, and Child's Play. I felt Nightmare on Elmstreet and Friday the 13th as a franchise were terribly overrated, and aged even worse.
Sometimes Silence of the Lambs gets counted as a Horror film - I consider it a suspense thriller; Psycho is another suspense thriller often labeled as horror - although it did have a profound influence on the horror genre. The worse mislabel I have come across is American Psycho as a horror film (and I think it's because of the cover and name alone) when it is a dark comedy that satirizes the yuppie scene, it has much more in common with films like Fight Club, Pulp Fiction, and Natural Born Killers - and is probably more light-hearted than any of them:
Here's the most brutal murder scene in American Psycho, it doesn't show anything except blood splatter and a swinging axe... but first, the setup clip for the context of why Patrick Bateman killed Paul Allen (Paul Owen in the book):
The film isn't horror, but it's still great! And I think horror film fans will enjoy it because we tend to have sick senses of humour =)