For me, there was never a “best part”, because my preferences got little consideration. But in those days (the very late '50s and the '60s) there wasn’t an extensive list of mandatory school supplies either. Often, the only things required were some pencils and a three-ring binder with some loose-leaf paper to fit.
Coloring supplies! Crayons, markers, colored pencils, pastels, paints, you name it!
New clothes, new markers and crayons, new lunchbox.
New clothes, well maybe socks and underwear, my older brother got new clothes and I got hand me downs

For me, there was never a “best part”, because my preferences got little consideration. But in those days (the very late '50s and the '60s) there wasn’t an extensive list of mandatory school supplies either. Often, the only things required were some pencils and a three-ring binder with some loose-leaf paper to fit.
New clothes - my mother liked buying new clothes for us!