I guess I'll be the odd duck and disagree.
I suppose it all depends on what you like/want out of horror.
For the best spooky atmosphere and focus on great acting/storytelling, it has to be the "Golden Age" of horror, with the great Universal classics of the 30s and 40s, as well as the Val Lewton series, and lesser known hits like White Zombie, Island of Lost Souls, She, Dr. X, The Undying Monster, Dragonwyck (great early Vincent Price film), etc.
The 50s and 60s had a lot of great monster movies, and spooky science fiction, as well as more supernatural horror. Movies like Them, Night of the Demon, The Blob, House of Wax, The Haunting, House on Haunted Hill, and even the original Godzilla, set a lot of standards for scary movies. And of course the 60s were stacked with great Hammer Horror and Roger Corman Poe films. The late 60s also saw the rise of more "psychological" horror, with films like Rosemary's Baby and The Devil Rides Out.
The 70s of course saw the rise of the so-called "Slasher" genre, with Texas Chainsaw Massacre, and Halloween. It was also a decade of more gruesome "nature goes mad" or "monster on the loose" type films like Jaws, Grizzly, The Swarm, etc. And the continuation of "psychological" supernatural horror, like The Exorcist, The Omen, Carrie, The Sentinel, and The Legend of Hell House.
The 80s seems to be remembered as the "Slasher Decade", due to the sheer volume of Halloween, Friday the 13th and Nightmare on Elm Street sequels that got shat out, along with the many, MANY copycat films that popped up. But there was also a lot more going on, horror-wise. I'd argue it was an even greater decade for non-slasher horror, such as The Thing, The Howling, Prince of Darkness, The Gate, Fright Night, The Lost Boys, Creepshow, Cat's Eye, Child's Play (the non-cheesy original), Critters, and arguably "hybrid" films like Gremlins and Ghostbusters, etc. There was a lot going on that decade, for sure.
Me personally, I'm split between the classic Universal "Golden Age", and the 80s. The 50s/60s "Silver Age" also has a strong argument, but there's something to me about that classic black and white era. I've never personally been big on slasher films, but I think the 80s produced an awful lot of non-slasher greats, as I pointed out. The 90s did have a few gems, most notably to me, John Carpenter's In the Mouth of Madness. But to me, the further we've gotten along, there's just something about most modern horror that is missing something that the older eras had. At some point, around the 70s and 80s, horror films started being less about spooky atmosphere and trying to tell good stories, to focusing on "shock value" and gore and body count, things like that. And I think that just evolved through the 90s, 2000s and onward, to a point where I've seen too many modern (meaning 2000s/2010s) horror movies that literally just try to be as disturbing and messed up as possible. I've seen many that forgo trying to have good acting, compelling characters, and genuine chills, instead just trying to be as gruesome, at times even sadistic, as possible. Films that are both arguably overly gross, but also just bleak and hopeless.
I guess my point is, why still focusing on being scary, a lot of older horror films from the 30s-80s, were fun. Like they were fun to watch, and yeah, bad things happened, and people often died. But at least to me, the BEST ones left you feeling glad you watched it. I think John Goodman's character in the 1993 Joe Dante film Matinee said it best, that the point of scary movies, is to take the audience on this ride, to distract them from the real world, and have them scared and on the edge of their seats, and then when the movie is over, it's like they can breath a sigh of relief, they get that catharsis from the experience. I just personally feel that with an awful LOT of modern horror films, you don't get that. At least I don't. There have been many modern horror films where I didn't feel glad for watching after seeing it, at all. And maybe that's me. And I'm certainly NOT saying good horror films don't still come out these days. But I do still feel like something is missing these days. *shrug*