As in... a song play an instrument on? None, though if I was in a rock band in general, I would want to play bass. But really, of course, I wish I could sing opera... any Puccini opera. I'd like to be a soprano who could handle that. Tosca and La Fanciulla would be the two I'd most want to perform in their entirety, but for arias, I think I most want to sing this one, from the last act of Manon Lescaut: "Sola, Perduta, Abbandonata." It is my favorite soprano aria by Puccini. No one could marry such despair and such beauty quite the way Puccini could.
(Senza Mamma from Puccini's Suor Angelica and the Seguidilla from Bizet's Carmen run close seconds.