Hm, I think a lot of the new episodes are way funnier than the old ones. Favorites are...

Casabonita
Lemmiwinks
Freak Strike
Awesom-O
Butters' Very Own Episode

and so many more...

But my absolute favorite was Scott Tenorman Must Die. Your tears are so delicious...

I've only seen the wallmart episode of the most recent season so far, and I thought it was funny.

I don't see South Park losing it's humor or appeal, I think it's changed a lot as it's gone on to remain hillarious for different reasons than when it started. It's great as a humorous and insane social commentary show that doesn't take itself too seriously, and can have new episodes come out fast enough that the jokes are still timely.