The second busiest place on the server I'm on is Gridanias Adventurer Guild.

There's at least 50 people piled in there at any given time, which causes the leve NPCs to take 2-5 minutes to spawn.

The reason why there's so many people in there all the time:

They're ALL waiting for the NPC to spawn XD

Ul'dah is technically 'the middle city'. To get from Gridania to Limsa or Limsa to Gridania, you have to go past Ul'dah unless you run the loooooong way round, or teleport. Because of that, among other things, such as the shops and guilds in Ul'dah, it will always be the most populated city unless they find some way to merge the market wards into one that you can access from all 3 cities, and make every single guild accessible from every city.