Favorite Opera : Lulu, and I'm tempted to say all time favorite music, period. Every month or so I dig out the splendid Schaefer DVD to watch a few select scenes... and weep along..
I've never been a huge fan of orchestral transcriptions, but Aschatz's one-piano transcription of the Rite on CD is fantastic indeed. Not so much technically (while it is also) but musically.
Ravel's La Valse, the 1 piano version again, which strangely I've always found better suited to the piano than to the orchestra. And I hate all the annoying glissandi in the 2 pianos version. Claire-Marie Le Guay has recorded a nice Daphnis & Chloé too, using a bit of studio re-recording. I say only "nice", cause I found it a bit tame at times. And the second movement sounds quite nasty on piano. Bleh. But ok, these two might not count really as transcriptions, since I understand Ravel used to compose everything on piano first, and then orchestrate.
I long for transcriptions of Webern's Passacaglia and Strauss Metamorphosen, 2 of my favorite works, NOW that would be something to play

Aah, sorry, I just realized I'm going a little off topic (not strictly opera).