Aside form the Human voice...

T-bone, I have the Channel Classics SACD, that is indeed a nice one, and I'll have to look for the Edgar Meyer recording. And if you haven't heard the Rachmaninoff, I think you might like it--the Steven Kates version on the Sonic Arts label from way back is a nice performance and recording (it's the one that introduced me to the piece), and there are some others out there as well.

French Horn (do you hear the fanfare?)! Combining a 4-1/2 octave range with incredible harmonic richness, a horn has a wonderful diversity of possible sounds. Brassy, lush, refined, romantic, stirring... What other instrument can migrate from the brass section in a band or orchestra to a woodwind quintet. You know you have a decent system if it can get the horn sound right. I tend to go for the classic Dennis Brain recordings, but there are many good players today, each with a unique sound.

rcprince, Edgar Meyer can make a double bass sound more delicate than I thought possible, especially in the upper registers.

2muse, I agree the French Horn is a nice instrument (and for percussives, you can always tap the side of a french horn with a drumstick)!