Probably the most famously dysfunctional family in the history of popular culture, the Simpsons are no role models, but are thoroughly redeemed by their obvious love for one another. George Bush might have encouraged Americans to be "more like the Waltons and less like the Simpsons", but we know who we'd rather hang out with.
Most Dysfunctional Element:
Homer's parenting skills probably fill this berth, although we'd argue that every time he throttles Bart, there's a good-natured attempt to bond with Lisa to balance it out. He tries his best, after all.
How To Make Them Functional:
Electro-shock treatment famously didn't work, and Marge and Homer have been to numerous psychologists and parenting experts, to little avail. Best let them get on with it then...