One of the funniest sitcom moments this year occurred during a recent episode of The Simpsons, when the cartoon clan welcomed an exchange student from Albania into their house.

The boy turned out to be an argumentative little commie spy. ”Now, now, Lisa,” flustered father Homer Simpson admonished his daughter at one point, ”you’re right — democracy is wonderful, but maybe our little guest has a point here when he says that the machinery of capitalism is greased with the blood of the workers.”

Holy cow! Try finding a punch line like that on Growing Pains, or even Head of the Class.

As a cartoonist who’s primarily a writer, Matt Groening is the reason the Fox network’s The Simpsons, on Sundays at 8:30 p.m., is such an enjoyable show. Groening has created a group of characters whose personalities and motives are more vivid and detailed than the vast majority of sitcoms featuring flesh-and-blood actors.

Take Bart, for example. Bart, with his head shaped like a grocery bag and his perpetually sullen stare, is the show’s break-out star. Bart is a wise guy, disrespectful to a degree that is already legendary. (The popular T-shirts featuring a bilious Bart muttering, ”I’m Bart Simpson; who the hell are you?” capture this tone nicely.)

But, of course, most of the kids on sitcoms are impolite little wise guys; why is Bart more interesting than, say, Pains’ Kirk Cameron?

It’s because Groening has invested Bart — and all the other Simpsons, for that matter-with a sensitive, vulnerable side that most sitcoms with human beings lack.

In the standard sitcom, kids are obnoxious, moms are long-suffering, and dads are dopes. They’re the cartoons; the Simpsons are for real:

*Homer is a dad trying to do his best, putting bread on the table even if it’s tainted with radiation from his job in a nuclear power plant.

*Marge is a mom who has learned to disguise her intelligence so as not to embarrass her huffily insecure husband.

*Daughter Lisa is a TV rarity — a smart, confident, sensible child.

*Bart is just as smart, but he’s rude because he doesn’t fit in with the world, which has already hurt his feelings a few times too many.

*Baby Maggie isn’t TV-baby-cute; she’s just there, all wide eyes and sucking noises, the way lots of real babies are.

A Fox press release says the Simpsons ”represent the American family at its wildest!”

Not true: The Simpsons are the American family at its most complicated, drawn as simple cartoons. It’s this neat paradox that makes millions of people turn away from the three big networks on Sunday nights to concentrate on The Simpsons.