Whatever I think in theory, when I am actually hearing some of the duets, trios and small ensembles from Mozart's greatest operas - these seem to me the height of musical achievement.
I am thinking of things like the parts of Act 1 with the three ladies and/or three boys in The Magic Flute, or Papageno and Pamina's duet; or Soave sia il vento from Cosi, or the Letter duet and Act 4 Finale from Figaro...
Of these, above all I perhaps love the little scene where Tamino and Papageno get their flute and bells.
I mean this section - albeit not this performance nor the recording quality:
Presumably this says a lot about me! but the lightness, the little orchestral touches and transitions, the harmonization of voices, the simplicity and sentiment, humanity and glimpsed divinity... well, only Mozart could do this; and perhaps this kind of thing is for me a higher achievement than any other in music.