From the 90's onwards, the following have been much admired:Amores Perros(the best i've seen),La Ley de Herodes (Estrada),Solo Con tu Pareja(Cuaron),Et tu Mama Tambien(Cuaron),Danzon(Novaro),El Mariachi(low budget big success),Principio y Fin (Ripstein).Many liked Cronos by Del Toro,though i wasn't so keen.And of course,Como Agua para Chocolate is the most popular Mexican film.
Of pre-90's films, try;Los Olvidados,El, El Angel Exterminador, Nazarin (all by Bunuel),El Compadre Mendoza(De Fuentes)-i've not seen it but it's considered a masterpiece - and Que Viva Mexico(by the great Eisenstein),both from the 30's.Gabriel Figueroa (Maria Candelaria)and Fernandez(Pueblerina)were also important Mexican directors.You could try El Topo(1971), by the Chilean director Jodorowsky, but Mexican,if you want something very unusual and surreal. 