Probably Ted the Caver for me. I feel like a lot of creepypastas go way overboard in trying to pull the rug out from under you in a gotcha moment or straight up just making an OC don't steal, but I always thought the more grounded and minimalist creepypastas did a better job.
It's a tie between The Russian Sleep Experiment and Psychosis, both inspired me a lot as a writer over the years. I keep coming back to how excellently tied together they are and yet leave you wondering about their universes.
MrBallen has a ton of spooky stories, probably too many to even pick a favorite, but this one is undeniably creepy. It's a true story about a cult-like family and the ending is just insane.