If you already decided on Asura, you should try the world form for Asura for linkshell information. Personally I don't believe there's a best or worst server, all servers are more or less the same in terms of population (obviously the newly merged servers will have more members for now, although it will normalize after a while), and population doesn't matter much in the first place. Every server has linkshells that accept new players and help them progress in the game, as well as an endgame-crowd.