Depends what your looking for.If it's english speaking people then the big 2:Diez and Ares are the ones for you.
If it's a quieter server,so you have a better chance of bosses etc,then Manes,Edana,Olympia are the ones for you.
If you want to play servers with lots of non-legit levels/items Cypher,beramus and logos are the ones for you.
I play Diez mainly,but also have char's on Logos unfortunately.