I know it really isn't very stylish or beautiful.
For a child(xs adult might work?), not bothered if it is for a girl or boy, preferably long sleeved but will look at anything that is logo free, no picures on it either.
I keep looking for one for dd, who is 9 in a few months, has no interest in clothes, but wants one. The one she has currently is a Next age 6, and she is now wearing it under short-sleeved t-shirts and vests (channelling her love of mini the minx, she loved Denis the Menace until they changed the style of it).