As a non playable race... Dragon, no contest. (I'm actually surprised not many people would want to be a dragon.)
As playable races, Khajiit because they are so amazing. They have fur, tails, cat ears retractable claws, night vision, they purr (I'm assuming), theres variety, and pure awesome, how can you resist?
I'd also go Vampire High Elf, because they are amazing at magic, and sorcery is by far my favourite play style, and vampire because a couple of weeks ago I fell in love with the idea of simply wandering northern Canada (or someplace cold, as vampires are resistant) with a black outfit and specially made umbrella for blocking all light. just for kicks and giggles.
(No, I'm not an twilight fan. I hate with all of my being people that portray vampires as super sexy never un attractive all sex and romance, and nothing about them being a predatory (parasitical) race of bloodthirsty hunters who actually slit you open and slurp your blood out 0_o )
Sorry, that got a bit long, but I hoped you liked reading my limb of my tree.