I love threads like this...scary things are always interesting.
In fact, I created a thread about this kind of stuff a couple of months back - I could place a link to it in this post, but I'm not sure if I'm allowed to do that or not. Either way, that thread kinda trailed off after a while, but it could be interesting if it was revived...
Anyway...
For me, it is the morgue scene in Night Terrors, and that episode as a whole, which seals the deal. A group of cadavers in death shrouds sitting up on their slabs might sound scary, but considering they don't do anything but that, it shouldn't be all too terrifying...except it is, and from my observations it's precisely
because they don't do anything other than sit up that it's so horribly creepy. Less, in so many cases, is more.
And of course, the whole premise of the episode - that is, being trapped aboard your own ship, which cannot move due to a mysterious alien phenomenon, with only a ghost ship full of people who went insane and brutally murdered each other for company, and no apparent way to escape, while also slowly beginning to lose your minds and become increasingly fearful of something truly unknown...I.E Worf's eerie conversation with Troi where he, the most powerful warrior and a Klingon, is terrified of "something out there which we cannot fight" (which, of course, isn't even real, and so the actual danger is of losing control of one's mind)...it really milks the whole "space can be frightening" thing brilliantly and horrifically.
I also agree with the OP about Identity Crisis. In terms of its scariness, creepiness and eerie, uncomfortable atmosphere, I think that episode is underrated. In some ways it's even worse than something like Alien, because anyone who's infected with that parasite will end up as nothing more than a creepy animal, wondering around in the dark on a desolate planet...forever.
Other than that, while Schisms is obviously scary, the episode "Remember Me" is quite terrifying in a psychological sense. Just watch it, imagine yourself in Doctor Crusher's shoes, and you'll see what I mean.
