I completely agree with you JasonDonnelly!
I played Machu Picchu in high school and the euphonium part was wonderful. The unison part with the horns during the climax was always a challenge (I was only a sophomore!). Thank goodness our horn section was stacked, so they nailed it every time. I haven't played Aspen Jubilee, but my high school did it a few years before I got there. I also own the score and am fairly familiar with it. All of the parts are wonderful in that piece! Another Nelson piece that comes to mind with a great (and hard!) euphonium part is Rocky Point Holiday.
If I was to add a piece to this thread that hasn't been mentioned, I would say a great piece with a really good euphonium part is David Gillingham's Heroes, Lost and Fallen (which I also happened to play in high school).
Kyle Aufderhar
1982 Besson Imperial 767 euphonium (Giddings and Webster Carbonaria)
1966 Salvation Army Triumphonic baritone (Doug Elliott, LT 100 rim, F cup, F3xs shank)
Lafayette Concert Band
Acadian Wind Symphony
University of Louisiana-Lafayette Wind Ensemble