Cheat alert. I'm not actually choosing an opera for 'Best Libretto,' but an actual librettist. As soon as I saw that this question was going to come up, I knew it would be too difficult to pick a favourite from Le nozze di Figaro, Cosi fan tutte and Don Giovanni, so given that the librettos for all of these Mozart operas were written by Lorenzo Da Ponte, can I just have them all? Thanks.
Da Ponte wrote twenty-eight librettos for various composers in the eighteenth and early nineteenth centuries, but his three collaborations with Mozart are the operas he is best known for. I think it is the collaborative effort that makes them truly special- the sparkling wit of the words meeting the sparkling music. There are so many stand out arias, duets etc from these three works. There are those that will make you laugh (Guardate!- Madamina, il catalogo e questo from Don G) and those that will make you weep (Dove Sono i bei momenti from Figaro) but I'm picking something that sort of does both, with a big helping of sex appeal on the side- Il core vi dono from Cosi fan tutte.
I absolutely love this duet between Dorabella and her lover's best friend Guglielmo, who is disguised as a mysterious Albanian gentleman in the hope of tricking her into cheating on her man (thus proving that no woman can stay faithful.) It's at this point in the opera that you realise that the ruse may be falling apart and a bit of a love square begins to form. Da Ponte's clever lyrics invite a certain degree of intimacy between the performers, and have led to some seriously sexy performances of Il core vi dono over the years (I can hardly bring myself to listen to Bryn and Cecilia's very breathy interpretation on their Duets album...) but I'm picking the Nicolas Hytner Glyndebourne production from 2006, in which Luca Pisaroni and Anke Vondung get up close and personal. 'Oh, cambio felice di cori e d'affetti!'