This type of project is right down my alley. I love horror films, and my only problem is going to be which films to cut from my list.
I'd like to give some love to maybe some under the radar picks:
Nosferatu the Vampyre (1979; Warner Herzog) - Herzog and Kinski team up again, this time in the most beautifully shot horror film of all-time. It's also one of the creepiest.
Who Can Kill a Child? (1976) - This film is the pinnacle of the evil children subgenre. The pinata scene is the best.
Amer (2009) - I am a big fan of art films, surrealism, and giallo, so this film is practically made for me.
The Phantom Carriage (1921) - IMDb lists this as horror, but I am not entirely sure if I agree with that. Regardless, if it counts, I'll probably vote for it. It's a masterful silent film.
Angst (1983) - A top ten pick for me, maybe top five. One of the most visceral horror experiences I've ever had, was this hard-to-stomach (should be) classic. It's in the vein of Henry, in terms of content, but directed much better. Just like Henry, it's an incredibly tense film.
The Beyond (1981) - I haven't been around this forum long enough to know how Lucio Fulci is considered here, so maybe this is or isn't an under-the-radar pick. Either way, it deserves mention. Plot wise, you can find better material almost anywhere, but as far as a surreal atmospheric trip goes, you can't do much better.
The Texas Chainsaw Massacre 2 (1986) - Obviously you should vote for the original TCM, because it's one of the ten best horror films of all-time, but you should at least give TCM 2 a fighting chance. It's a lot less scary, but it's definitely a fun and at times creepy ride.
The Prowler (1981) - As far as slashers go, this is one of the best formula slashers. If you don't like slashers, you probably don't need to see this.
Hellbound: Hellraiser 2 (1988) - This is sort of the same situation as TCM 2, except I like this more than it's original film. It has a really cool depiction of Hell.
House (1977) - The less said, the better. You just need to watch this film, because there is nothing else like it.
The Innkeepers (2011) - I don't know, I might be alone on this, but I thought this was a really tense ghost story film that oozed atmosphere.
Christmas Evil (1980) - The best Christmas horror film of all-time. It's almost more of a character study than a full-blown horror film, but it's very effective as a depiction of a man's descent into madness.
The Sacrament (2013) - Two Ti West films and no mention of House of the Devil? I actually haven't seen it, but this film is another really tense film that has a subject matter that interests me. That might make me like it a little more than I should.
Possession (1981) - A serious contender for my top spot. I think people fall into two camps for this film: those who think it's brilliant and those who think it's silly. You just have to watch the scene with Isabelle Adjani in the subway, I think it's some of the best acting I've seen and others think it's completely ridiculous. It's an amazing film about the feeling of separation, a pretty universal feeling among any person who has ever had any kind of relationship.
Excision (2012) - AnnaLynne McCord gives a very surprising performance, and is very good as the very disturbed main character in this film.
Jason X (2001) - Okay, not under-the-radar, but definitely underappreciated.
Ghostwatch (1992) - I was going to ask whether or not this was eligible, but it looks like somebody earlier in the thread already mentioned it. It's the Blair Witch Project before the Blair Witch Project, but in a totally different setting and with less shaky cam.
Jigoku (1960) - Perhaps the greatest depiction of Hell ever on screen. It's up there with Dante's Inferno from 1911 and Hellraiser 2 as my favorites.
Blind Beast (1969) - This movie is fucked up.
Knock Knock (2015) - It's not necessarily good, but it's a lot of fun. It's a fringe candidate for my list.
The Babadook (2014) - The best film about grief being manifested as a babadook I've ever seen.