Official by far. Most active playerbase, most stable server. Only downside, it is P2W until C28 (or C30 depending on your budget) or so, then you can do whatever you want, even with shit SPs. If you can pull through and get there, you'll have the best experience you can get for this game.
Private servers aren't worth playing. They forgot they tried to be Nostale and made custom content, so much it looks like Nostale, just doesn't feel like it.