I've been watching horror films since the late 60's... Had a terrific beta collection of movies, then a terrific collection of VHS movies and now a big collection of DVD's. On top of that I subscribe to Shudder and would rather watch a horror film over any other genre.
Here's my best of the best:
"Dawn of the Dead" the 1978 version. Flat-out the best zombie film of all-time IMO. The level of gore for 1978 was stunning. It moved the bar as far as what and how much gore was being shown in movies. But the movie was more than gore it had a great story combined with interesting characters topped with George Romero's bent towards making a social statement. The placement of most of the movie in a mall was brilliant. Today we may not realize how powerful that was, being that malls are dying out, but in 1978... Malls were at the height of their impact on society. Did I mention how great the gore was? Finally... the first 15 minutes of the movie isn't just a great horror movie beginning, it's a great movie beginning period.
"Halloween" again the 1978 version. Absolutely love this film. It doesn't have the biggest body count or the most gore– As a matter of fact most of the kills are rather tame by today's (or even the original "Dawn of the Dead's" standard). But the story is so incredibly simple and powerful. It affected me so powerfully because it reminded me of our "trick or treat" nights in the 70's. The characters were great and I became smitten with Jamie Lee Curtis ever since.

On top of all that the soundtrack was brilliant– John Carpenter is brilliant in so many ways.. The theme became an instant classic.
"The Thing". Another John Carpenter created classic. The special affects even today are amazing. The desolate setting helped make the film. Kurt Russell was great, as were many of the actors. Loved the story about a shape-shifting alien. The "tour" by McCready and Doc through the destroyed and abandon Norwegian camp had brilliant atmosphere. While "Dawn of the Dead" and "Halloween" were recognized as terrific films when they were released, "The Thing" was treated by critics and movie goers as a "red-headed step child". I was one of the few that ventured to the theater to see it. Today, "The Thing" is recognized as a classic and deservedly so.
"The Shining". My gosh I've watched this film so many times, and I'm still left in awe on how great this film is. There are so many interesting tidbits to this film... Of which folks are still debating to this day. It's one of the few Stephen King books where the movie adaption was better, which seems funny because Stephen King dislikes Kubrick's version. The actors? Jack Nicholson's portrayal of Jack Torrance is of the all-time great acting jobs no matter the genre. The "Overlook" hotel location was brilliant and served as another strong "character" in the movie. The ending of the movie is one of the all-time best.
"Godzilla King of the Monsters". While Godzilla turned into a comic-book style hero in the late 60's & 70's, the first movie, made in 1954 is mind-blowing. And to think where Japan was after WWII to come up with the story of Godzilla and pull off a movie like this– Can't give them enough credit. If someone had to watch one kaiju movie this one should be it. The Japanese version with subtitles is a tighter production than the Americanized version with Raymond Burr. What makes this Godzilla movie stand out is that it truly is scary.
There you go... My all-time top 5.