I always get a kick out of these large boxes. 
I like the colors. It looks like a gay pride banner.
"What is a good 'unknown' work to hear?" asks Brett.
The problem is, of course, that if I know the work, it's not unknown, and I have no idea who else knows it. So I don't know what works Brett does not know. But if the issue is, what Mozart works are worthwhile ways to upend the cliché that he writes shallow, boring music, I could offer:
- Quartet in D minor, k. 421
- Quintet in G minor, k. 516
- Mass in C minor, k. 427
- Piano Quartet in G minor, k. 478
- Rondo in A minor, k. 511
- Prague Symphony in D (#38)
- Symphony #39, E flat, k. 543
- Piano Concerto in G, #17, k. 463 ?
- Piano Concerto in A, #23, k. 488
- Piano Concerto in C minor, #24, k. 491
- Piano Fantasy and Sonata in C minor, k. 475/457
- Clarinet Quintet in A, k. ??
- Don Giovanni
Just for a start.