Genji is a lethal offense hero in "Overwatch" and he can be a one-man wrecking crew if used correctly. This hero is not only very mobile, but his attacks are strong, he has the ability to deflect attacks and his ultimate ability is one of the best abilities in the game.
Considering that it takes a while to master Genji, he should be only used by skilled players. If you decide to use him, make sure you memorize the tips below as they will help you go up against every hero in the game. The tips were brought by KarQ and one of the best Genji players in the world, shadder2k.
One general tip you have to remember is that you should always use secondary fire before using any other abilities as it increases your overall damage with no downsides.
Offense heroes
Use double jump to avoid Doomfist's Rocket Punch while he is charging it. When going up against another Genji, try to deflect his attacks but remember that you can dash through him if he is deflecting. If you are close to McCree, make sure you keep jumping as it will be harder for him to land a headshot if he stuns you with his Flashbang. Try to avoid Pharah, but if she is not flying high, you can try dashing and doing some damage.
When fighting against Reaper, deflect his shotgun pellets as it can kill him easily.
Also, make sure you deflect his Death Blossom. Get close to Soldier: 76 using your dash and land a couple of hits, but don't bother deflecting his Tactical Visor. If you are hacked by Sombra, you can still climb walls, so use it to your advantage and escape. Try to time your deflection when Tracer starts shooting to force her into using her Recall.
Defense heroes
Jump around Bastion to make it hard for the hero to land shots on you and use your deflect if you get hit by it. Don't jump too much in front of Hanzo as he can easily hit you and kill you that way. Use your shurikens to poke Junkrat from distance then dash to finish him off. Also, dash slightly upwards to avoid his trap.
Don't get too close to Mei to avoid her freeze, but if she does start to freeze you, use dash to escape it. Furthermore, you can deflect her ultimate ability. You can easily 1v1 Torbjorn, but his ultimate ability is a big counter for Genji, so make sure to not use Dragonblade during it. Don't use your deflect ability as soon as you see Widowmaker as she might not be looking at you. However, do use it when she's aiming at you and ready to shoot.
Tank heroes
Fighting against D.Va is easy, so make sure you poke her to build your ultimate charge. When around Orisa, jump and dash around her to use her immobility against her. Keep hitting her while doing it to quickly take her down. Use wall climb or a double jump to avoid Reinhardt's ultimate ability.
Also, deflect his Firestrike to send it back through his entire team.
You can block Roadhog's hook with deflect, so make sure you also use it when he hooks your teammate as you can just dash towards him and use deflect. Avoiding Winston is easy if you dash away from him. Also, you can kill him in a 1v1, but you must land headshots with your shurikens. Wait for Zarya's bubble to run out before using Dragonblade as it can block two slices of your ability.
Support heroes
Ana is very hard to kill as Genji so make sure her abilities are on cooldown before getting close to her and engaging her in a fight. Lucio is a good counter to Genji since he can boop the hero during Dragonblade. Make sure his boop is on cooldown before using it, or just don't make him your primary target.
If you have your ultimate ability and going against Mercy, first use dash, then secondary fire and melee before activating your Dragonblade. This way, it will take only one more slice to eliminate her. Also, don't use your dash if she hasn't used Guardian Angel as she can easily escape. As a flanker, Genji should locate and destroy Symmetra's teleporter and turrets. You can take her turrets down by using primary fire and flicking to each one of them. Finally, you can easily 1v1 Zenyatta and you can also one-shot enemies during Transcendence if you are nano boosted.