Atterburg 8. It really doesn't get much better.
Gliere 2, Kabalevsky 2, and (though not an unsung, the piece is) Rimsky-Korsakov 1 also come to mind.
Gotta love those Russians I guess...
Oh and I guess Myaskovsky 19, though it's for Wind Ensemble. And for something a bit more modern but still fully in 'our' idiom, there's James Barnes's 3rd...though as far as wind ensemble pieces go that's probably not all that unsung.