To hard to choose, for myself I think.
Anything that isn't human. Don't get me wrong, I love the humans (Imperials especially) but speaking as a human I love the opportunity to play something that isn't one. The aliener the better. The more distinctive the better. Shame everyones non-human heads are still stuck on oh so human shaped bodies, but I'm patient.
Argonians and Khajiit I'll never get bored with (oh to one day have a game set in a Argonian or Khajiit dominant region) plus they look amazing, I have always, always liked Altmer and Dunmer (and finally, finally a game in the ES series that gets the alieness vs humaness of them right) - the aliener the better. Bosmer used to make me sad, but they are so good now (looks wise and play wise). Haven't played an ork yet, but again - they have hit the nail on the head with orks, so soon.