I think your question is well, a bit, subjective.
Personally, I think Goblin is a great server with a pretty cool community!
Then again, it also depends on what you are looking for. For example...
Raiding? Gilgamesh and Leviathan are pretty good for that I hear.
Balmung has good reviews, also it is the unofficial RP server.

Also look into what hours you have available to play. That could help you pick a suitable server.