I think season 1 is a little too crude to embrace entirely (due to the primitive animation and less developed writing), but it's not terrible, and definitely better than about the last 12 seasons. Season 2 and 3 are better, but still a little clunky in pace and lack the sophistication of some later season writing/humour, although it has its share of bonafide classics (i.e. "Lisa's Pony").
I consider seasons 4 to 7 "The Golden Era", when just about every episode was great, and the episodes that weren't great were at least very good. They started experimenting a little in season 8, resulting in a few episodes that were a little disappointing, but I don't think the show got consistently bad until season 9.
I bought seasons 4 to 8 on DVD. I was hesitant about 9, so I rented it, and it didn't take long for me to realize that I made the right choice not buying it and it is definitely when the show started running out of ideas.
It's the first season (after about 5 in a row) that I thought had consistently bad episodes and did some absolutely atrocious character assassination. What they did to Skinner in "The Principal and the Pauper"
was shameful and after 7 seasons of loving him dearly, I think "The City of New York vs. Homer Simpson"
was the first (and unfortunately not the last) episode that made Homer annoying and painfully grating to me.
I like to pretend everything after season 8 never happened. I wish it hadn't, even though there have been a handful of really good episodes since then (i.e. "Girly Edition"
and "Trilogy of Error"