Mexico City's best neighborhoods for tourists are Condesa, Roma, and Polanco — safe, walkable, and full of great restaurants and cafes. Centro Historico puts you near major sights. The Airbnb market in Mexico City is excellent and often beats hotel value.
In the Yucatan, Merida makes a great base for exploring cenotes and ruins; Valladolid is charming and cheaper. Oaxaca's small colonial center means almost any hotel puts you within walking distance of everything. Beach destinations (Tulum, Puerto Escondido, Sayulita) range from backpacker cabanas to high-end eco-resorts.