Oh man, Tom and Jerry is a gold mine for this sort of thing. If it even counts as a sound effect, I'd definately go with Tom's trademark "AAAAAAAAH!" scream. It NEVER fails to crack me up.
But otherwise, I love the sound made when a character is hit by something, usually a plank of wood or a window shutter. Sounds more like a gun going off, but it works so well that it's hard not to smile.
There's also the tremendous explosion sound they used when Butch smacks Tom in the backside with a guitar. Y'know, just to let you know that it hurt.
Ed, Edd n' Eddy has several that I like, though it might be because they're so bizarre. Like, if a character peers over a fence for example, you'd get a faint sheep 'meeeeh!' or something.
My favourite one is pretty hard to desribe though. It's usually used for when Ed is stumbling around or has his head trapped inside something, and sounds like a strange mumbling noise. If you've ever seen the episode where he's working in Rolf's shed at peeling a potato, you'll know the one I mean.
There's also that one they use for when a character is about to let loose a heavy blow, and sounds vaguely like a stereotypical martial artist. "Zudolo-zudlo-za-za-zuuudLA!"... something like that, anyway.
And finally, while I don't know where it originates from, I love that 'yodeling' sound used when a character reaches a lofty place, or even falls from it.