The Clarinet Quartets? The Requiem?
The Requiem never did much for me. I am unfamiliar with the Clarinet quartets. The clarinet is not one of my favorite instruments though (granted I prefer it to the awful flute).
The Posthorn Serenade is wonderful.
Mozart's Clarinet Quintet is awesome. I'll never forget David Shiffren playing that at a small brunch/concert over a decade ago. It was one of my first live classical concerts, and I was hooked. That clarinet was so sweet.