The server is good, RMT and no dual box is pretty good but the server is very unbalanced...you can enjoy it until mid game, but you got no late game mass pvps. You either join the top side, which farms all epics and has castles with box clans even, or you have no late game.
For me anything from interlude to High five. The first classic is playable too.
If you dont have L2 experience go for late High five perhaps. The problem with interlude is the files are almost always crap, and its much much better on later chronicles.
For me from Interlude to High Five is the best. From there it went downhill. Maybe the first classic is playable, other than that I really like to stick to gracia/high five. Interlude is a bit outdated, but for old players brings good memories.