Personally i think Family Guy is bloody hilarious, albiet a lot of the jokes aren't exactly high-brow. Someone also implied that Futurama's a very funny show- i beg to differ. I love Futurama, but at best the show makes me chuckle. Family Guy has me laughing my ass off.
I have also come across some pretty funny American stand-up comedians, Reginald D Hunter comes to mind, he's done live shows in the UK.
Americans can be funny of course, like any nationality- personally though i think Britain and Ireland does humour better though.