I really like the Westin Galeria Plaza... right in the Zona Rosa, they often have specials just about $100/nt. Its full-service, but not too large (I prefer it over the huge Sheraton Maria Isabel just on the other side of Reforma at El Angel).

In MEX, it is important to stay near where you really want to be at night, because you can't take regular Taxis at night UNDER ANY CIRCUMSTANCES, and the Sitios (special taxis that are at hotel stands) may be hard to come by at night. So this means you don't want to stay "centro", leaving you with Polanco (the most expensive hotels in town, and some nightlife, but not the greatest choices) or Zona Rosa (lots of moderate-priced hotels, lots of restaurants (very affordable) and clubs, shops, etc. And convenient for your daytime activities. (2 metro stations nearby, Chapultapec nearby... do not hesitate to take the Metro which is one of the world's finest... clean, quick, safe(ish), and SUPERCHEAP. Under US$0.20 anywhere in the city.

I will stop my discussion now....