US-Area 52 is very horde heavy, and a high pop server as well.
Here's a list for US servers.
Illidan 17,043 (1/0.00)
Mal'Ganis 12,436 (1/0.01)
Azralon 11,246 (1/0.05) - never even heard of this server lol
Area 52 15,350 (1/0.03)
Other servers will be lower pop and much higher ratio. These 4 are your best bets.