Season 1: The best of all. I dont think any show has ever produced a perfect season of television but if it was possible, this would be it. The atmosphere, storytelling, and characters were all new at this point and it was a great intro to the story. Music was also the peak here. My opinion might be biased based on nostalgia for the time I watched it.
Season 2: A great follow up to Season 1. They kept the same atmosphere and were loyal to the characterization. Good character development this season. Love the Halloween setting, and again, nostalgia for the time in my life that I watched it.
Season 4: Arguably the most epic in scale, I like the callbacks they made to past seasons and tied all the storylines together. Vecna is by far the best villian, and it's crazy to know he pulled all the strings from day one. When will is possessed in Season 2 he draws the same shadow monster Henry does as a child. He also speaks about the mind flayer as "him" but we now know this whole time hes talking about Vecna, since Vecna and the Mind Flayer are linked.
Season 3: I honestly think this whole season is filler. Its like an anime where you have Manga episodes but theres some filler you can watch that doesnt really progress the story any further. However, I do think this season is the funniest out of all, they really peaked in comedy here. The action sequences are cool too. The only thing that adds to the story is when Billy is talking to Eleven in a vision, he is talking through the Mind Flayer. But his mannerisms and tone of voice very much resembles Vecna, which we know was him this whole time now.
Conclusion: I like how this show is realistic in terms of we only know what the kids and adults know until further revealed. The group had every right to believe the main villian is the Mind Flayer, because there was no way to discover that Vecna was behind it. Until Vecna was infiltrating psychic contact, which he got from Elevens leg, and until Eleven went to Project Nina.