honestly anything works...it depends what u plan to do and ur playstyle/preferences
troll have the slow reduction, so if u hate getting kited this could be nice...also berserking is an okay move when u are taking dmg
tauren have more hp and another interrupt...perfect for arenas/locking down healers
orc have +5 axe skill and bloodfury...they do the most dmg
undead have basically a fear immunity every 3 min...and the shadow resist helps avoid fear...if u hate fear go UD