I'd rank seasons 4 and 5 slightly above 6
I think 4 and 5 are pretty interchangeable in terms of the series' best season (4 might be more consistent but I think 5 includes more of my personal favorites) but 6 is slightly weaker, though not by much. My main problem w/ it is that the premises started to become more gimmicky and less based in reality ("Lisa's Wedding", "The Springfield Connection", "Lemon of Troy"), which points the way towards the shell of itself the show would eventually become. Fortunately, the show is still about as funny as it ever was and includes a few of my favorites, like "Itchy & Scratchy Land", "Homie the Clown" and "Lisa on Ice". Sorta splitting hairs though- again, I own seasons 3-9 and never get tired of them and still consider it my all time favorite show.