Altmer. Mostly for the physical appearance (height) I'm a pretty tall person in real life so I find the immersion factor of Altmer the best for me. Furthermore I love playing wizards and Altmer make the best wizards in Skyrim. Culturally they are all kind of jerks though.
Altmer, followed by Snow Elves, and then Ayleids.
Nothing like playing an Altmer warrior. The height is stunning. The only race, IMO, other than Orcs, who don't look derpy in armor. I'm playing an Altmer paladin on Requiem. He started with an increased cost to power attacks and a 50% weakness to magic. He's now a total badass. None of the drawbacks of being a warrior, all the benefits of being a mage.
I actually find their culture very interesting. Read Rising Threat (a four-volume set on the Great Anguish and its impact on the High Elves) and there are several excellent articles here on their culture. It will put a whole new spin on the Altmer. Thalmor is not synonymous with Altmer. They are arguably one of the most tragic races in all of Tamriel.
Nope? Come on, explain! The Falmer, no matter how awesome they look and how interesting they may be, were quite literally everyone's bitch. The Nords hunted them for fun and the Elves used them as slaves. Tell me, how are they the "master" race. Just curious, maybe you're even right, who knows.
Aherm, let's dig into the lore.
Context: Et'Ada created more-or-less all deities and everything. Among those were Magnus, the Aedra and Daedra, and more that you may have heard of. Then there's Ehlnofey, who you've probably not heard of. He's the one who created the races; Men and mer. Among men were the Nedes, Yokudans, and Amorans. (Ancient Imperials, Redguards, and Nords). All other races, except for the Breton and Argonian, are Elves. The argonians spawned from the hist, and the Bretons were created through years of crossbreeding between elves and men. The "regular" elves; Altmer, Dwemer, Ayleid, Snowelves, Maormer, and Sinister, are just different types. Maormer are lake-elves, altmer are islandelves (basically the original) and Dwemer are earth-elves. Then there's Orsimer, who were fucked up by Malacath and Boethiah, which made them look like mutilated elves with a skin disease, and the Dunmer, who were originally Chimer before Azura got her hands on them.
Finally, Bosmer and Khajiit. One day, Azura and Yf'fre got angry at eachother and decided to screw around with some random elves they found. Azura took some and put them in the desert, made them VERY versatile; that's why there are so many types of Khajiit. Yf'fre, however, was less original, and just put them in a forest and made them look kinda like trees; Bosmer.
Point is, if you're going to nag about Khajiit not being mer, Bosmer wouldn't be mer either. They had the same history till a god changed them. Same with Dunmer and Orsimer. (sorry for wall of text :/
I'll get you some links, so you can check for yourself if you want to.