For a singer, there are many of Mozart's shorter pieces like the "Laudate Dominum" that are a pleasure to do, but my absolute favorite is the "Requiem" which I have had the privilege of singing in twice in my life. What a thrill! It's one thing to enjoy listening to a great piece of music, but to study it for performance is to really live it.
I also was the stage manager for the "Magic Flute" done by a local opera company once, that was a different (hair-raising!)experience but no less enjoyable.
For simply listening, "Eine Kleine Nachtmusic" is always good.