i used http://www.wowprogress.com/realms/rank/us as a guide when I resub'd, it lists realms by PVE progression. I picked Blackrock since its #2 in PVE progression in USA, is a PVP server, has very strong horde presence (im horde), is in battlegroup 9 (which people say is the most competitive), and has a very short ping time for me in the west coast (20-30ms).

A high pop server is nice because the economy is very vibrant and good progression is nice because its very easy getting into Kara, heroics, and 25 man pugs. I've run a couple of sunwell trash farming PUGs and every day there are mag/gruul pugs, and on weekends a few Hyjal and Naxx pugs as well. I would have never seen Sunwell on my older PVP server