Who should i main

Try each of them out in quickplay and see which one you enjoy playing the most. Alot of people will tell you to play the hero you're best at, and that's good advice, but you should also play somebody you find fun and enjoyable to play.

Main a character is good, but you also need to learn how other characters works, because sometimes your main is not the hero that your team need.

It's a personal prefrence. Do you have good aim? Then you should try sometrhing like soldier or macree

you main the one tank that fits your playstyle the most

Orisa will probably bring the most utility to your team. I find Roadhog is better when playing soloQ though or if your team supports are... lacking. Since the self-healing, hook and dmg output will cause you to win most 1v1's.