As noted in the other thread, my very favourite piece is Allegri's Miserere. The authentic voice of God.

I'm also extremely fond of Ungar's Ashokan Farewell, used as the theme for Ken Burn's Civil War documentaries. Haunting and beautiful.

For reflective moods, I love Albinoni's Adagio in G minor. Such longing, such deep melancholy. I have a recording of it with trumpet rather than organ, and it is sublime.

Of course, this is from a man who collects Requiems.