there really is not 'best' race to play, just like there is not best class to play.
the asslings have 1 advantage over all other races in the game though. they get a 5% xp bonus. this means it will take them 5% less time to level, thus every 20 levels they get 1 level ahead of any other race out there if hunting the exact same mobs in the exact same groups in the exact same zones for the exact same amount of time.
they will level 20% faster then iksar and trolls as those 2 races have a 15% xp penalty due to their regen rate.
that means an assling will every 5 levels get 1 ahead of a troll or iksar.