On the Aether data center, Balmung and Gilgamesh are the most populated - with creation of new characters restricted at all times on the first and almost as much on the second. However, server population isn't going to help much. Crystal Tower is an outdated raid with no incentive for revisitation. Until they implement a reason to run these again, you'll find queues that can last for days no matter what server you're on sadly.

I have a newer alt that I'd love to get through that particular questline just for the cutscenes but I've pretty much given up any hope of having that happen.