When I was a kid/teenager, the station to beat was 102.5 WZBQ on the "Tuscaloosa tall tower" west of Birmingham. I believe it 73 kW at around 2062 feet AGL and at the time there was very little elsewhere on the dial to get in its way. The gentle rolling hills of the lower Appalachian mountains to the east meant it suffered from some pretty severe terrain shadowing in the Birmingham metro and points east, but if you stuck with it, it'd be a fairly decent signal to well east of Anniston. After that point, the 102.7 licensed to Fruithurst (playing to west Georgia) would slop over it, but then once you got past that station, 102.5 would waft back in and make it almost all the way to I-285 in Atlanta, a distance of about 160 miles.
Headed south, it was good all the way to Greenville, Alabama (~120 miles) thanks to favorable terrain, along I-65. The west and north were the less impressive directions; I never remember hearing it much past the AL/TN line on I-65 or west of Tupelo (95 miles) in Mississippi. It was practically a local in Meridian, though. This would have been ~1994 or thereabouts.
Not too long afterwards Clear Channel flipped the station to a country music format, then moved it into Birmingham-proper, with much lower power and HAAT. The era of it being a big signal was over. An honorable mention big FM signal would go to WZZK in Birmingham. It seemed to be the best-performing FM signal actually in the market, with a lot of resistance to multipath that plagues some of the other signals.
As for AM, the ultimate drive-able signal for me was always the daytime signal of WCRV in Memphis. 640 kHz, 50 kW and ground conductivity that was pretty darn good meant it was a rock solid signal from Jackson, Mississippi to Ste. Genevieve in Missouri, roughly 200 miles north and south from the TX site. If not for 630 in St. Louis I'm certain it'd make it partly into that metro as well.
When I lived in central Mississippi, even with a broken antenna/amp on my car, WCRV was an easy catch at 100 miles out. There were plenty of places in rural Mississippi where it was the only viable thing on AM with that gimped radio setup.
Speaking of St. Louis, that city has some of the most robust and "drive-able" HD signals of any market I've been in over the years. Granted, the sheer distance one achieves with FM HD is nothing compared to the analog, but I remember all the FM HD being robust and completely dropout free — with nothing more than a portable radio inside a car — all around the city on the I-270/I-255 loop. Several of them were strong enough to stay (mostly) locked all the way to Hermann, Missouri, about 60 miles from the sticks. Again, not impressive by regular analog standards, but still some of the best HD I've gotten with the little Insignia portable inside the car.
An honorable mention goes to my local iHeart country station "95 KSJ" in Mobile, who operates at higher HD power and has a good I-10 signal between Crestview and Pascagoula, about 60 miles in either direction.