This question is more for the AM (MW) and FM broadcast band DXer, but it might also apply to shortwave radio. Which station is your closest unheard station?
On the AM side, my closest unheards are both in Illinois. For the life of me, I have never heard WJPF 1340 Herrin or WDQN 1580 Du Quoin, both 100 miles from my shack in Hazelwood. During the daytime on 1340, I primarily hear KXEO Mexico, MO (with a full-service Adult Contemporary format) or WSOY Decatur, IL (with a news and conservative talk format). One of the disadvantages I see is in the station's power. WJPF only operates with 770 watts; KXEO and WSOY both operate with 1,000 watts. Except for three hours a day, their programming is simulcast on WCIL 1020 in nearby Carbondale. 1340 at night is a hodgepodge of several stations, with WLOK Memphis, TN most frequently poking through with its Urban Gospel format. Other 1340 stations that poke through frequently include the aforementioned KXEO and WSOY, along with WCDT in Winchester, TN, KROC in Rochester, MN and KDTD Kansas City, KS. That would make hearing WJPF a difficult proposition at my location.
By contrast, WDQN only broadcasts with 170 watts daytime, and very low power at night. Even with WBBA Pittsfield, IL off the air, hearing WDQN is also a difficult proposition. First off, I have a local during the day on 1570: WBGZ Alton, IL. The station does provide mild adjacent channel interference to any station on 1580. In recent years, the addition of digital service to another local, KATZ 1600 St. Louis, MO, has made things much worse in the interference department. It's made any attempt to hear WDQN on the AM side much more difficult. I'm very lucky that I've heard WDQN-FM on 95.9 MHz many times in the past, even after flipping to Christian programming from the Three Angels Broadcasting Network (3ABN). It's also the only Illinois station I have NOT heard on 1580; I've heard the now-silent WKKD Aurora and WBCP Urbana in addition to WBBA. Hearing WDQN on 1580 is just as difficult of a proposition.
In the state of Missouri, where my shack is located, my closest unheard on AM is KDEX 1590 Dexter, at 140 miles. The digital operation of KATZ 1600 makes hearing the 620-watt daytimer an impossible proposition. During the daytime, when the digital interference from KATZ is minimized, WAIK Galesburg, IL (5,000 watts during the day) is dominant. There are a few stations in the Bootheel of Missouri that's difficult to hear because of adjacent channel locals or more powerful signals closer to the area. For example, one of the reasons why I haven't heard KMIS 1050 in Portageville (600 watts daytime only) is because of a 1,000-watt station only 110 miles away (WDZ Decatur, IL).
On the FM side, my closest unheard is KDBB 104.3 Bonne Terre, MO (just 60 miles away). This is due to the fact that I have a local on 104.1 (WHHL, although it's licensed to Hazelwood, has their transmitter in University City) and a semi-local on 104.5 (KSLQ Washington, MO). WHHL's digital operation makes matter much worse, although I've nulled it out to hear stations like KMCR 103.9 Montgomery City, MO or WOMC 104.3 Detroit, MI. Even with my location being 17-25 miles from most of the local high-power FM stations, I have some trouble hearing adjacent channel stations through the digital interference. The only 100 kW station that does not broadcast in digital format is KLJY 99.1 Clayton, MO (with a Contemporary Christian format), which makes hearing FM stations on 98.9 a bit easier, although I have some splash on 99.3. I can bring it down enough to the west to hear KCLR Boonville, MO.
What are your closest unheards on AM (MW) and FM? If you include shortwave, that closest unheard on that band could be one thousands of miles away.