Depends if you are NA or EU. If EU I would suggest Ragnarok, if NA I would suggest Balmung or Gilgamesh. Every server has friendly and deuche people, but being on more populated server is definitely an advantage, as you will have less problems finding FC, endgame static, friends etc..

ATM all servers are located in America, but that is going to change, so if you are EU you definitely want to play on EU servers.