I'm going to play the Aztecs first, unless I talk myself out of it and change my mind 1800000 times between now and late Sept. Which seems likely. But here's my reasoning:
1) In Civ 3 and 4, the only thing worse than playing the Aztecs was playing adjacent to the Aztecs. How you supposed to learn with those fools on your borders?
2) In Civ 5, the Aztecs actually look to be useful and enjoyable to play as, with a clear early game strategy of "Kill all the barbarians and use that to buy tradition/honor social policies that make you better at fighting" coupled with a late game strategy of "Kill everyone on the map and use that to buy the autocracy/order social policies that make you better at fighting."
3) I really liked that Aztec mod for Civ 3 with enslavement.
4) I can win a cultural victory just by bulldozing my enemies? Yes ma'am.
I'm also going to use my psychic powers to say that my normal Civ group (best friend, his girlfriend) will play Rome (because he pretends he's Italian) and China (because she always plays as girls, regardless of stats).