BTW, right in my town, there's a 5,000,000-watt
UHF TV station, but that's the ERP due to antenna
gain, not actual transmitter power.


5 MW UHFs are pretty common, really.

Here's a real challenge: list the top ten (or top five) highest gain AM
stations.

That is, those with the highest maximum-to-RMS field ratio, regardless of
power.

The power ratio, hence the gain, is the square of the field ratio.

Hint: start looking at stations with eight or more towers.

Second hint: look for arrays with major axes of 180 or somewhat more degrees
and with minor axes of 90 degrees, more or less (210, or so, degrees and 80, or
so, degrees, respectively, are among the prime candidates).