Unless you're a top 20 world guild, pick the one you like visually or that has an activated racial you like. The dps difference in the racials ESPECIALLY in WoD is so marginal that unless you're pushing a world first kill it won't matter.
For live as of this moment: Orc/Pandaren are pretty close. I personally like Orc for the Fist Wep expertise buff and the on use AP to macro into TeB.
For 6.0 and beyond: Orc/Troll/Tauren I'm hearing are going to be pretty close. I'm probably going Tauren so 1) I can be fucking massive, and 2) Super ToD's