Teike's Old Comrades is #1 on my list but in my favorites are :
Hermann Blankenburg's True Comrades in Arms & Ernst August...
Franz von Blon's Under the Flag of Victory & Sounds of Peace...
Nowowieski's Under Freedom's Flag
Just a few, all have soaring and prominent euphonium lines. There are so many good marches out there with nice euphonium parts. Colonel Bogey (particularly if the saxes don't play!!), Alford's The Standard of St. George, Fucik's Children of the Regiment & The Florentiner so forth and so on.
As for Sousa, well he didn't write very many nice marches with the euphonium in mind, did he? Nice Marches, but not as much fun to play as those of the German style.
Give me any march with a soaring euphonium line that stands out above the band.