S1: 6/10. Fear of the unknown, quick glimpses of the monster, well timed jump scares, and main characters almost dying. The demogorogon being able to make portals really worked to keep the characters and audience on edge that it could show at any time. The Bad Men being an intimidating threat and empathizing with El's fear of Brenner.
S2: 8/10. The MF being this gigantic alien that is right on the other side of our dimension. It terrifying Will and hounding him. Will being powerless to stop it possessing him.The audience not knowing how the gang could stop it until the final episodes.
The sound design in the first few episodes conveying that the characters are on edge. The demodogs growing more dangerous and the uncertainty of whether the kids would make it with so many demodogs running around. Owens explaining how the MF is like a virus. When Hopper was stuck in the tunnels and attacked by the vines reminded me of some terrifying/suspenseful times playing video games.
Billy's temper feeling like it could go off at any second. I was really worried about him going after Lucas in the school parking lot. The whole craziness at the lab and Bob going by himself to turn on the power. Bob forgetting the gun...aahhh. The horror that Brenner could be alive.
S3: 6-7/10. The scares really kicked in mid way, but there was a lot of good build up with the rats, Billy's possession and him abducting Heather and almost getting Karen. I was seriously worried about Karen there. The anxiety of what the Red Army could do if they managed to open a Gate.
The Scoops Troop being stuck in the underground base while entertaining was also scary in how defenseless they were if they got caught. The horror themed music this season was the best hands down. It's awesome each season, but they out did themselves this time as it really amplified the intensity of the horror, action and gore.
The whole hospital chase scene was extremely well shot and cut, and I totally felt Nancy and Jonathans terror. I've had nightmares like that. Grigori being this hulk of a person that could beat the cr*p out of Hopper was scary in that he seemed unstoppable. I wanted him as far away from Joyce as possible. The stakes getting higher as the season progressed, and after El got badly injured the fear that other mains could also get just as injured or killed kicked in.