I swear by concert D.
Easiest notes for brass, especially horn.
Never taught by rote for longest time in the beginning stages. But went to a workshop about ten years ago on starting beginners.
Taught D,C, Bb with out music. Hot Cross Buns, Mary Had a Little Lamb,and Clair Delune.
Was able to focus on posture and sound.
After D,C, Bb then we go up Eb f G. Also did theory and note names in the process. When we got to the book. It was a breeze.