I'd probably head for Zocalo Square in the Centro Historico.

http://www.delange.org/Zocalo/Zocalo.htm

Taxis are available after you exit customs and all the way to the right.
I'm sure you'll get all types of advice about the taxis in Mexico City but I avoid issues and use the "official" airport or hotel taxis. Sure I pay more but I've never had a problem.