It seems to be split between Balmung and Ragnarok (and a fair bit of EU people on Ragnarok used to be on Balmung before the server split). If you are an EUer (and play in evenings) then Balmung would be a safe choice.

It also has a vibrant JP and NA community.
It also is home to the RPers.

If you want to do some real reserach some options are:
-- Check each world forums, and see what kind of responses you get.
-- Create a character on the worlds and see what it is like.

Of course, this may alllllll change in a couple of months when we are all reborn.