Top 4 choices imo.
Cabal Online = Fairly active people, not real hard to level, hard bosses, hard dungeons near end game, amazing skills.
Atlantica Online = VERY active game, TONS of Active people, plays like a console turn based RPG...and world is HUGE, based on the REAL world (map wise).
Perfect World = VERY large scale world, lots of active people, flying is fun, semi good leveling.
Granado/Swords = Pretty hard early on without help because of the lack of money, but good with help, and tons of content.