Mainly the early ones that I haven't seen in years, but I am rewatching some other later seasons also. Pity my local library doesn't have season 6, that one had a lot of episodes I liked.
I still stand by my statement ages back, South Park only got and continues to get better as it goes along.
Considering I don't want to steal Baal's thunder on that long report he did listing his favorite episodes and explaining why, I'm just going to list mine quickly.
Season 1 - None, this season was just meh and I didn't have a favorite among them.
Season 2 - Chickenlover
Season 3 - Korn's Groovy Pirate Ghost Mystery
Season 4 - Do the Handicap Go to Hell?/Probably 2-parter
Season 5 - Scott Tenorman Must Die
Season 6 - Tie between Free Hat and Red Sleigh Down
Season 7 - South Park Is Gay!
Season 8 - The Passion of the Jew
Season 9 - Trapped in the Closet
Season 10 - Make Love, Not Warcraft
Season 11 - Fantastic Easter Special
Season 12 - The Ungroundable
Season 13 - Dead Celebrities
Haven't seen any of Season 14's first half yet. It hasn't aired here, probably will soon.