Aida
The music of the opera is excellent all the way through.  There are quiet songs of incredible beauty as well as big chorus numbers.  The music is all famous, but among the most well-known parts are the aria that Radames sings near the beginning of act One in which he dreams of victory in battle and marrying Aida, the Ethiopian slave.  The song is called Celeste Aida ("Heavenly Aïda").  The Chorus in Scene ii of Act II, Gloria all'Egitto, ad Iside ("Glory to Egypt, to Isis!") is one of the most famous marches ever written.