There have been a number of great solos played very well on euphonium. Many for trumpet, trombone, horn, cello, and even violin. One time before a band rehearsal during warmup I played, "I'm Getting Sentimental Over You" (Tommy Dorsey's theme song). I got a queried look from one trombonist who said, "Please don't".
Last week I heard a recording of George Michael's, "Careless Whisper" - one of his most famous pieces. It starts out with a gorgeous tenor sax solo played by Steve Gregory. I found a simplified chart online then entered the notes into Finale and tweaked it some (took chorus up an octave). It plays okay but it's not the same. I'm usually not crazy about saxophones but its voice is perfect for that piece.
I wonder what other music we shouldn't attempt to play?
-- edit to add mp3 from Finale ---
MP3 of Careless Whisper (midi out from Finale):