I'd say 2 or 6. 2 for reasons already stated, 6 because, well, I think it had a nice, kind of different atmosphere than a lot of the other season.
And definetly not 1, the characters were still developing and they changed too much that season. Three was pretty okay, but I kind of thought that some of it was boring. I'd say 4 and 5 were also okay, some of the characters/ plotlines becoming too predictable, though. 7- also okay, some of the characters were beginning to get flat though- Kimmy's annoyingness (If that's a word), Joey's acting like kid too much- they were just getting annoying. And season 8- I didn't very much like season 8. The characters seemed kind of dry, and while they were good in many eps, it wasn't enough for me to like it as a whole.