MK: Pirates, Tangled, Pinocchio are usually quieter. Tomorrowland, Adventureland are bad. Midday the Casey’s bathrooms can be quiet too.
Epcot: Imagination bathrooms are almost always empty, great front park option. The Norway ones are very nice, but more popular. If you’re in Le Celier their bathrooms are very quiet and rarely used.
DHS: RIP gonzos, but I’ll throw out a shout out to the secret backstage bathroom in Rise. Back when that was new, I did not realize how long I would be waiting and was in pain by the time I got to the indoor queue. The CM in the line took me back and then after much relief I was able to rejoin the queue at merge.
AK: Great everywhere, although the Everest ones are the most crowded.
USO: ET, Potter, MIB are my most frequent places. MIB is horrible during HHN though. The Gotham Ice Company ones are so hidden I even forget about them.
IOA: Besides the ones mentioned, I would add in a favorable review of the Namor bathrooms (obscured from guests walking towards Toon Lagoon, the POE bathrooms by Seuss, and pretty much all the JP bathrooms (not just the pizza ones, but River Adventure and Visitor Center) for being not well utilized.
Epic: Not enough experience yet, haven’t had a bad one yet.
Volcano Bay: The bathrooms that are least used are the River Village ones by Krakatoa, because it’s furthest from the enterance and wave pools.