Short answer
We cannot say with certainty that Le nozze di Figaro (The Marriage of Figaro), K. 492, was Mozart’s single favourite opera. It was, however, one of his greatest achievements and a work he clearly valued highly.
Why Figaro seems especially important
- Personal investment: Mozart showed sustained enthusiasm for Figaro while composing it (letters of 1786–87 record pride in the music and delight in working with Da Ponte). He lavished care on ensembles, character writing, and the score’s dramatic detail. (See Deutsch, Mozart: A Documentary Biography.)
- Artistic breakthrough: Figaro marks a major development in Mozart’s mature operatic style—integrated ensemble dramatics, psychological nuance, and sophisticated orchestration—making it a milestone in his output.
- Professional significance: The Vienna premiere (1786) consolidated Mozart’s reputation; he conducted, revised, and returned to the work, indicating ongoing regard.
- Collaborative bonds: His close working relationship with librettist Lorenzo Da Ponte and the original singers strengthened his investment in the opera.
Why we should not label it definitively “his favourite”
- No explicit declaration: Mozart never left a clear, contemporary statement naming any single opera as his favourite. His correspondence praises various works at different times.
- Other contenders: Don Giovanni (1787) and Così fan tutte (1790) are treated—both by Mozart’s contemporaries and modern scholars—as equal masterpieces; Mozart spoke warmly of them as well.
- Interpretive evidence: Later biographers sometimes infer that Figaro was his favourite, but such claims rest on interpretation rather than an unequivocal primary source.
Scholarly consensus
Musicologists typically place Figaro among Mozart’s supreme operatic achievements while avoiding a categorical claim that it was his personal favourite. Useful references: Otto Erich Deutsch, Mozart: A Documentary Biography; Maynard Solomon, Mozart: A Life; Daniel Heartz, essays on Mozart’s operas.
Conclusion
Le nozze di Figaro was clearly one of Mozart’s most beloved and important operas—a work he cared for intensely—but the surviving documentary record does not support a definitive statement that it was his single favourite.
If you wish, I can quote specific Mozart letters about Figaro or point to passages in the scores/arias that show his special musical attention.