Thing is what are your goals when you play? I reckon folks are trying to have fun in their own way so please don't include that. Let's see if I can help:

1. Do you want to/have been trying to raid? If so, high populace servers have better chances of getting you in a static or for you to build your own.
2. Are you a roleplayer? If so there are actually two servers highly regarded as being RP servers. They are Balmung and Gilgamesh.
3. Free Companies are a preference. Myself, personally, like a mix of raid serious/casual friendly willing to help when the need arrises. You need to look for what suits you once you transfer.