From experience, I can say that the best PVP realms for Alliance are Kel’Thuzad, Tich and Kiljaden I think. For Horde you have a bigger list to choose from. If you take a look at wowprogress, most of the top 20 realms are PVP realms which is surprising. But you will notice that Horde tends to Dominate PVE even on PVP realms. However, you will find a bigger pool of PVP talent on the more popular realms like MalGanis, Illidan, Blackrock, Tich again and KJ, and even KT.