I would say an under served market would be Lafayette, IN. They only have 2 Class B signals in their market, & a few Class A stations that are in their market (Tippacanoe County). The few Class A stations that are outside of Lafayette have tried marketing themselves as Lafayette station, even if they're not always heard throughout the market.
Chicago has got to be the most overserved market. Almost every frequency .4 spaces apart has something on it within the Chicago market. The non-commercial band is too complicated to list, due to many Class A & D stations (especially in the 88.1 - 89.9 range). I'll only focus on the commercial band: 93.1, 93.9, 94.7, 95.5, 96.3, 97.1*, 97.9, 98.7, 99.5, 100.3, 101.1, 101.9, 102.7, 103.5, 104.3, 105.1, 105.9, 107.5 are all that broadcast from the city of Chicago. 101.9, 102.7, 105.1, & 105.9 are licensed to the suburbs (101.9 to Skokie, 102.7 to Oak Park, 105.1 to Evanston, & 105.9 to Elmwood Park, but marketed as Chicago stations), & 97.1 is the only station licensed to Chicago that doesn't reach the northern suburbs well (especially Lake & McHenry Counties), due to 96.9 transmitting from Pleasant Prairie, WI. Class B stations 92.3, 106.7, & 107.9 are Class B stations that have coverage over Chicago, but 92.3 (licensed to Hammond, IN) only covers the southern portion of the market, & 106.7 (licensed to Des Plaines, IL) only covers the northern part of the market. Only 107.9 (licensed to Aurora, IL) is located in the western suburbs, & able to reach nearly the entire market, including the city of Chicago. 99.9 is a Class B that was originally licensed to Kankakee, IL, but the COL was changed to Park Forest, & only coverss the south suburbs. The rest are Class A stations that are located in different parts of the market. Before I forget, 96.9 is a Class B that originally served Kenosha, WI, but sometime in the 70's, or early 80's, the COL was changed to Zion, IL, & simulcasts 97.1, since 97.1 doesn't cover the northern suburbs well (especially Lake & McHenry Counties in Illinois).