I have just recently transferred from Goblin (Sep 2013-May 2015) to Gilgamesh (May 2015-).
Goblin is "smaller" population server with some fantastic FCs for social, casual players. I would recommend either Dark Lords and Infinite Theory.

If Goblin was so great, why did I leave? The answer is Raiding availability, the average US prime time night on Goblin has 3 raid parties in the PF and like 8 total.
Gilgamesh on the other hand, being a large population server, has about 10-15 raid parties in the PF and near 20-30 total all the time, day or US prime time.
The FC I am in on Gilgamesh, is also very friendly, Equivalent Exchange. I do not know enough about the server culture on Gilgamesh to comment on that yet.