I think that one would be a tossup between WKZC Scottville, MI, WOLX Baraboo, WI or WUPZ Chocolay Township, MI. On radios with less-than-DSP-sharp selectivity, watch out for 95.1 WLST Marinette, WI, blasting 100kW from 16 mi NE.
How about ...
1090 kHz - at the KAAY Transmitter Site, while KAAY was off the air earlier this year / late last year (or whenever it was). Assume you're using KAAY's towers as your receive antenna (or something with equivalent gain), connected to a very sensitive receiver (one that could clearly get a signal without any antenna connected, that a Superradio, CC Radio, PR-D5, etc. can't detect with their built-in ferrite bars).
Question for Scott Fybush, in advance of one of my future ideas. What are some high-power (~50+ kW TPO) FM stations (preferably non-IBOC) atop high-rise buildings, either with public access to the roof, or another nearby (within 0.05 km) building or hilltop on the same level with the antenna's main beam?
One of my ideas would be first-adjacent to that local, for example 90.5 or 90.9 on Mt Wilson, CA, or 100.5 or 100.9 at the Soledad Cross in La Jolla / San Diego, CA. I'd like to find some that are a few decades of dB stronger though - preferably good enough to completely swamp a Tecsun DSP radio across all bands with the antenna disconnected.

As for the AM side along similar lines, I was thinking 1490 kHz near the center of the Northlake Festival parking lot in Tucker, GA, while WSB was still running IBOC, or, 1080 kHz (or 1071 if you want to try a TP/TA split) near the southwest part of Columbia Park in Torrance, CA, before KNX started using IBOC.

(You wouldn't have to use WSB's or KNX's tower as your receive antenna, but bonus points if you could get stations *ON* the target frequencies.)