An interesting idea would be to look for the counties with the greatest range between their two most partisan precincts. Obviously the winner hands down is Brooklyn, which has some almost 100% Obama precincts and some almost 100% McCain/Romney precincts, and most of the next runners up would all be in the Deep South, but it'd be interesting to see the ones otherwise. St. Louis County is a great example of a really diverse politically county for one.
Harris County:
969: Obama 411 (98.6%), Romney 1 (0.2%), Undervote 5 (1.2%)
969: Sunnyside: Orem, MLK, Almeda-Genoa
1030: Romney 439 (87.1%), Obama 54 (10.7%), Johnson 7 (0.8%), Undervote 4 (0.8%)
1030: Spring: Kuykendahl, Gosling, Rayford, Willow Creek
266: Romney 526 (48.0%), Obama 526 (48.0%), Johnson 20 (1.8%), Stein 3 (0.3%), Undervote 20 (1.8%)
266: Channelview/Cloverleaf: East Freeway (I-10), Uvalde, Greens Bayou
