Iowa, hands down. MN, WI, IL, MO, NE, and SD all complain about IA drivers. Blue plate specials.
Illinois drivers = Fifi's
F'n Idiots From Illinois
Then the cars stopped on highway entrance ramps almost always have Kansas plates. Don't know why...
SD complaining about Iowa drivers? That's sort of like Mao complaining about Stalin killing his own people. We used to joke that all the surrounding states - except Illinois, which seemed to produce nothing but snotty people driving as fast as possible in the straightest line - were full of farmers who learned to drive on tractors, thus the low speed, lack of turn signal use, and general confusion when they'd get "to the end of the row" only to find out that they couldn't simply spin the knob.
In northeastern Iowa, all you hear is complaints about the clods from WI and Illinois; when I was a kid in northwestern Iowa, it was complaints about hicks from Minnesota and South Dakota (a Hawkeye considering a Gopher or Dakotan a "hick" is one of life's ironies). Not content with interstate bashing, people from Humboldt County (IA) used to complain about the clueless drivers from Calhoun County, and I still think that the county I presently live in is as bad as Iowa gets. Upon reflection, it is sort of terrifying that so many other states are ranked lower than us.