While I am not a teacher and never taught in my life (on a pro-level anyway lol), I think Snape was a
realistic teacher/professor. I don't remember I've had a teacher who did not have a niche or had their unfair treatment to some students or...etc. (maybe this is more common in developing countries? IDK), but in general, I'd assess teachers based on their technical knowledge rather than their personalities and mental packages.
I think Snape is my favourite character in the franchise because of how realistic he acted and felt, how relatable he can be to some. And from what was written about him in Potions, he seemed to be a good professor.