The use of "piece" is a bit confusing. I guess it would help to clarify whether you mean any ballet music, or: (1) scores of complete ballets; (2) individual pieces from those scores; (3) pieces famous for ballet dancing that were originally written for some other context. Anyway, here are three of my favourites, one of each type.
1. One of my favourites is Tchaikovsky's score for the complete ballet Nutcracker.
2. Within that score, one of my favourite pieces is "March of the Toy Soldiers."
3. One of my favourite famous pieces for ballet dancing is "The Swan." It is in the suite for instrument ensemble Carnival of the Animals by Camille Saint-Saens, and was choreographed later on.