It really depends what type of "gameplay" you're into.
Era is a modern server but if you're just starting off now it may not be the best decision to play. Obtaining money and/or items can be a difficult task for newcomers. I've played UN a few years back and I was a huge fan of it, however I haven't recently played so I can't really express my current opinion on the server.
Graal Kingdoms is a fantastic server but it's also hard for a newcomer to begin playing and obtaining items. I've heard different things about Zodiac and I haven't really played for some years so I can't help you there. Classic is preparing for a re-release, you could check them out, I've seen some great updates for it so far
You also have "UC" servers but eh~
Keep in mind you may (well you more than likely do) need some sort of subscription to fully experience these servers. Anyways, pick your poison