humans and nightelves are great for dueling.
i would personally say that nightelves are overall best because the shadowmeld gets aggro off of pets and provides a ghetto vanish.
humans have a quick trinket as well as perception. however i'm not sure if the swords skill actually works in wsg/arena as having a low weapon skill in bgs doesn't seem to matter.
gnomes do have escape artist which can be nice and they have 240 engineering which gets them a couple of different uses (idk what though

). as nicke said escape artist can be used with insignia which can allow for great chasing and flag carrying abilities, but they aren't great overall imo.
dwarves used to be good until the stoneform nerf. now i'd say they're worst overall for alliance.