If you like winning, and you want to be a Royal Guard, from what I've heard, it's better to play on Venus (unless you play a Medic and you can get in good with ViciousVix, in which case, Andromeda might be better).
If you want to be a Free Knight, and you think you can be a really good player, or you just want a challenge, play Valkyrie (watch out for your allied guilds, FairyTail and Motorboat - they like to steal everything).
If you want to be a Royal Guard, you like a fair even faction with few "pro" players ('pro' is subjective), and/or you like a challenging opposing faction, try Valkyrie.
If you like a challenging opposing faction, and want to be a Royal Guard, try Valkyrie or Andromeda.
If you want to be a Free Knight, and you like a challenge, try Venus.
If you want to talk to people from the same server on the forums, choose Valkyrie, or failing that, Andromeda.
As far as popularity goes, I would say that Venus is the least
popular. For some stupid reason, Aeria thought that they should try 2 servers during OBT (as opposed to CBT), and they took down the best server (name), which was Venus.
They eventually got Venus back, but most of Venus' players couldn't wait, and had moved to one of the other servers by then. However, the recommended server is the one with the least players, and it changes from time-to-time. Like I said, who knows?