I think, the best server that trends right now has these things:
Good Economy(Players can maximize the loots, quests for money purposes(zeny, items, etc)
Balanced(Non Donator vs Donator wherein, donator has an edge but still, can be defeated by talent)
GMs are not bias (GM should be fair to everybody)
Active PvP Room (Pvp room should be always active or there are always players that play inside the pvp room)
Active WoE (players always prepare things needed for the woe, or playing woe for something or etc)
Active/Friendly/Rivalries Community (one of the purpose of guild war is rivalries, some guilds fight for glory, or they want to prove something,)
Less Bugs (in a server, bugs cannot be avoided, but could be lessened.)
That's all, if you have something in your mind, feel free to add them.