… to be fair, while I found the idea simply great, I was not the one to actually prepare this list I am talking about. Kudos to C.P. Dunphey (editor of the publication Gehenna and Hinnom) to have taken the time and listed them together with offering comments. 43 movies, to be precise. Some of them are foreign, all of them pretty good.
Here you have my favourite picks:
Ken Russell’s Altered States (1980). I agree with Dunphey’s comment: “The film explores concepts way ahead of its time, possibly considered one of the first metaphysical science fiction movies. The classic theme of science gone wrong is at play here, and with a standout debut performance from William Hurt, there is not much more to ask for.” Just a note: Ken Russell also directed Gothic (which is not on this list, but it should be). Never heard of this one either, where David Byrne plays Lord Byron? Go and watch it now! 
Sunshine (2007). This one comes from another well-known director, none less than Danny Boyde. Not as famous as, say, 28 Days Later, Sunshine is chilling, almost disturbing in some scenes. And the plot is pure apocalyptic SF. I guess it will be even better received now than at the time it came out.
I found on this list also other movies that, while not among my darlings, have been of enough interest for me either to write reviews (Pandorum, 2009) or to watch their remake (Flatliners,1990; the 2017 remake is nowhere as scary as the original, though). Neill Marshall’s The Descent (2005) is here in the UK rightly famous, at least the first one (I don’t advise you to watch Descent 2, which almost turns in parody a good horror flick).
And Terry Gilliam’s Twelve Monkeys (1995), well… I don’t agree it’s underrated. It is a cult movie in its own right and all SF lovers -who somehow are a different audience from horror -know, and love it.
Finally…Angel Heart (1987). I know this movie by heart and the performance of both Mickey Rourke and Robert De Niro is remarkable. Who wouldn’t work for a client called Louis Cyphre? A noir-horror set in Louisiana that gives you shiver.
I could add more, but at this point I really suggest you go and read the list here. You’ll find some pleasant surprises.
I love horror movies but have to confess there’s a few on the list i haven’t come across. The Descent does make my all time faves list and I’m also a big fan of Sunshine, Flatliners and Twelve Monkeys although it’s ages since I watched them.
Thanks! I found Flatliners one of the few cases where both original and remake are good, even though the first is creepier.
I am delighted to see Angel Heart on this list. It’s been years since I watched it, and I haven’t thought about it for a long time, but this post brought it all back 😀 The Descent is also brilliant.
Angel Heart scared me so much the first time I’ve seen it in the theatre. So creepy… when I visited New Orleans I kept thinking about it. 😀
Twelve Monkeys is the only one I’ve seen on your list, but it is a corker. And I would like to see Gothic though I’ve yet to see it included on the terrestrial schedules. And does Brazil count as horror? Certainly the last image of that nice man Michael Palin scared the bejazus out of me…
You’re right, and funny you mentioned Brazil. I was on the point to include it in the list…but Brazil is a cult in its own right, so ‘underrated’ was not completely appropriate IMO. Gothic on another end has never the fame it deserved. Shame 🙁