This is a nosleep story, so don't shoot the messenger if you're just looking for stories on this site. The Ocean is Much Deeper Than We Thought. https://old.reddit.com/r/nosleep/comm...
I don't know if you'll find it super scary (it's a damn good story), but I'm a real sucker for wtf stories set in far-flung places like outer space, the bottom of the ocean, or Antarctica. Anything with a vibe similar to John Carpenter's "The Thing".
The Dionaea House was one of the first creepypastas I read that evoked a feeling of genuine dread within me. The original website is only available via the Wayback Machine now, but there's a text-only version on here.
As far as the classics go, IMO it doesn't get much scarier than The Yellow Wallpaper . The idea of someone holding so much power over you that they can keep you locked inside a room until you literally become insane is horrifying. Oh, and The Curse of Yig !
I don't know if it would be considered SFW, but the scariest creepypasta I've read is "He Was a New Man" by HumboldtLycanthrope. It's a fictional story, but it discusses lobotomies and their history by both professionals and amateurs. There's no escape from the horror anywhere in the story. I may be the most squeamish person on the site, but I'm not the only one who found it a disturbing tale.