South Park is the steady decline of funny, if you ask me. It doesn't bother to try to make sense at all, it never has, and it just isn't that funny. I mean, it's like Oprah's Vagina has a gun, oh that's classic comedy.
I never really watched Futurama. I think I did once, and it was okay, but I don't really remember it.
Odd. I thought that was Two in a Half Men.
In all seriousness...or not, after you watch the same thing for a long period of time, especially comedy, you get a bit dulled by it. The Simpson's were funny once upon a time, but I liked watching it when I was in middle school. We were all a bit goofy in middle school, so I liked it. Family Guy really gave me a laugh, I haven't watched a recent one though. I never liked American Dad, Futurama, etc. If I'm looking for comedy, I'll usually give a look at some new comedic high school anime. They're usually purposefully ridiculous and insane, which is why I like those better since they don't take themselves seriously. Maybe it's odd to say that since we're talking about comedy cartoons. They eventually get into a rut like every TV show does, fail to be original again, and stagnate. Probably why I prefer my cartoon comedy in anime. After 20-some episodes its over. No questions asked. XD