It's now 13 seasons in and has only become a better show. Many shows stagnate and run out of ideas after a couple of seasons, but South Park continues to improve. I've loved South Park since it first came out, I think I was about 11 years old, it was pretty funny to me then but looking back there was so much I was missing out on. It's the same deal with The Simpsons, much better when you're older.
South Park is an amazingly current social critic, the episodes are produced each week in their entirely. Not the way Family Guy or The Simpsons is made, where each episode takes a total of approximately 6 months to create. With the ability to produce episodes so fast (I believe they even produced an episode in a day) this also allows the show to be fresh. If you look through an episode list you'll see the names of episodes that make fun of all sorts of current fads and fashions - Make Love Not Warcraft, The Passion of the Jew, Chinpokomon to name but a few.
The characters are brilliant, Cartman being an obvious favourite, he's extremely selfish, conniving and often cruel. Stan and Kyle are great a contrast to Cartman, level headed and sympathetic to others yet still retaining the qualities of the children they really are (see - Good Times With Weapons). The cast of characters extends far beyond the main four, a few favourites of mine are Butters, Randy Marsh and Mr Garrison.
Something else I enjoy but isn't really funny is at the end of a lot of the episodes there is a message or moral, in amongst all the laughs there is actually something you can take from the show. Watch the episode - With Apologies to Jesse Jackson, and you'll see what I mean. Fortunately this isn't something that's the main focus of the show, it's just a little something that might only take 30 seconds then your back to the laughs, you can take it or leave it.
At the end of this current season there will be a total of 195 episodes and it's still Comedy Central's highest rated show each week. South Park is basically the reason why Comedy Central is what it is today. I believe South Park is the best animated comedy series ever made. There is a lot of fuss over The Simpsons, and rightfully so, it was a great show for many seasons and paved the way for shows like South Park. But for South Park to still be going strong for as long as it has, for there to be no end in site, that is truly something awesome.