Mine personally is the isle of conquest.
It is awesome because you can wreck members of the opposite faction and rack up tons of honor kills. One time, our whole alliance team just stayed in our keep, let the horde knock down our walls, and we defended our leader for about an hour before our team finally broke through their gates and we ended up winning by having our team kill theirs. Was epic.
Also whats not to like about parachuting in to the enemy keep, using bombs, blowing stuff up with tanks, catapults, war glaives, flag captures. So much to do in that battleground.