For me season 3 is scariest because it was the most gory with the rats and people melting into blobs. Flayed Billy was also scary in how much control the Mindflayer had over him.

Season 1 i found the best, when i found out about it when it came out i watched in one go till 3 am and have been rewatching it periodically cause i found it so good! The whole vibe, the music and the acting is phenomenal.

I hate to say it, but i find season 4 the worst as of now. It was all over the place, the 2 days later skip????, sad Robin did not get any real time with Vicky or a conclusion atleast, the Venca reveal felt anticlimactic, unresolved story line from Enzo amongst others, Joyce just up and left the kids, how bad Will was treated and how it ended for Eddie. Still not over that... AND how in the beginning D&D seems to be super important but seems to be forgotten, but only being mentioned for a short bit to try and find out who 001 is in the D&D universe as Vecna. However, i did like Argyle and Jonathan helping the kids and being funny in general which i did not expect. Max her story line with Steve and the boys was immaculate, the scene at the graveyard with the music montage and the Dear Billy part really gave me chills.