Season 3 is probably my favourite because of the significant increase in comedy. For me I felt that the writing, stories and characters had developed seriously enough in the first two seasons, for them to be "allowed" to noticeably deviate from the format and ramp up the comedy levels, including the probably deliberately unbelievable main premise of that season, given the writers self confessed influences from 80's drama/films. That premise was surely chosen to further enhance the comedy fantasy for that season and I personally think it worked. I also just finished binging the four seasons and for me season 3 looks to be a one off. I was aware some people looked down on season 3 before I watched any of ST, so when I started season 3 I initially viewed it with some suspicion, but that very quickly dissipated once I could see it was going for comedy above all else. It has also been great to see the Steve arc, and season 3 is where it takes hold.
Edit: It is worth mentioning that they have a comedy genius on their hands in Gaten Matarazzo/Dustin, so the comedy fits particularly well with his "group" in S3, though for me the funniest scene was Detective Byers. Gaten Matarazzo was also responsible for bring me to tears for the only time in the whole thing for *that* scene at the end of S4 by the way, he is the star of the show for me, serious acting chops all round.