I prefer Fauré's chamber music. The violin sonatas, in special the first, the superb piano Trio, the string Quartet, the two piano Quartets and, above all, the two piano Quintets, to me the absolute masterpieces of Fauré.
Regarding the songs I am not so enthusiastic. Some are beautiful, "Au cemitière", "La Bonne Chanson", "Mirages", but in general I prefer the songs of Duparc, Chausson or Ravel.
In the piano music, the best are the Nocturnes and the Thème et Variations.
The opera Penélope has very beautiful moments (almost all the 2nd act), but I think it has not the power, the dramatic urge that the subject would deserve.