He doesn't really have a weakness except heroes like pharah. Tracer has been good and Genji is a very forgiving must pick hero.

Genji's mobility, lack of needing skills to aim, dash burst, deflect, overall surviveability, and easy to use team wiping ult makes him a must pick in many situations. Countering him with most heroes is pretty hard if he plays well. He's much harder to take down than a Tracer because of his animation, mobility, and deflect.

Junkrat w/ 2 bombs and zone denial makes it a lot harder for genji to get in close and harass people. He can blow up Tracer. Bust down groups on objectives. Also has an easy to use team wiping ult.