A quick look at the seasons show me that my favorite ones are season 5, 6, 7, 8 and 9. The more you go back in time from season 4 to 1, the less good the episodes get. From season 10 and onward, there are still great episodes, but not in such a density as in 5 to 9.
As I said before, last season had 5 episodes (200-201 and the coon trilogy) that I really didn't like. I just didn't care for the story (which is important if you end episodes with cliffhangers) and the humor was also pretty meh. I'm with the people who thought that the Mr Tenorman storyline and Kenny's death storyline were not funny or all that clever. But more than those 2 things, I remember being annoyed by the whole Cthulhu-Cartman-Totoro thing. It made me cringe. I also agree with the people who think that recent episodes won't age as well because the cultural and current events references are now way more pronounced. Insheeption was more of a remake of the movie instead of just "taking inspiration" from it.
But even though SP right now is not as good as it used to be, and there are some stinkers here and there, I still thoroughly enjoy watching the show. It's still a very entertaining series, and in no way do the newer seasons go off the deep end like other series do/have done in their later seasons (like, the Simpsons).