just ask yourself what hero would you recommend to a friend who can’t play the game.
If your friend is a aim god but cannot do anything besides clicking heads give him widow.
if he won’t survive anything give him moira.
if he feels like the game holds his skill back give him tracer.
if he likes spamming and plays in bronze give him junkrat, nothing more fun that getting rewarded for bad enemies than junkrat.
if he is bad and wants to improve and play multiple heroes I would suggest something thats easy punished to teach him like zen.