Franz is the newest server, and pretty well populated in starter areas, but it's on Euro friendly times, so you'd need to decide if that would work for you for sieges and such.
Bartz is also heavily populated, but not so much activity in the starter areas since it's the oldest server; most of the players are higher level. I don't have much experience with any of the other servers, so I'll let others fill you in on them. :)
what about the server with the most pvp cuz im on franz right now but i dont know i mean if i get into a clan that does sieges and crap since im from the us they might not be such a good idea and as far as franz with automated characters ya thats basically 99 percent of the people on franz(excluding me)
i was talkin one day to someone on franz to and he said there was more pvp on bartz so i dont know