Apparently I'm only good with Bastion, Mercy, and Soldier 76. I'm unsure how to feel about this. I can't Genji for the life of me, and although I love D.Va I can play her about as well as a cockroach plays an accordion.
Flankers like Genji and D.va [and reaper and tracer] really thrive on knowing the map better than their opponents, and knowing their hero matchups pretty well. So the more you play as other heroes, and the more familiar you get with maps, the better you'll start doing.
Genji, though, is entirely a beast of his own. I've seen a couple streamers with 200+ hours of Genji, and they all play him a bit differently. The way I see it, you play Genji until you find out what works for you, you find a nice rhythm of being aggressive, using Deflect, and Swift Strike. And eventually, you become an incredibly hard to kill squishie ANNIHILATOR. The couple key things you wanna work on, though are
1: Core combo: Right click>melee>swift strike. You can cancel each into eachother, and when you're fast enough you shouldn't even see your shuriken or melee animations, but they'll still be there. This combo does upwards of 135 when executed properly, and that's just body-shotting one shuriken.
2: Incorporating double jump and wall climb into your strafe. Once genji's deflect is down, he has no damage mitigation. So jumping and climbing around like a madman is how you prevent people from landing shots on you. You want to make it so that, if enemies are around, you're just double jumping and climbing erratically like second nature.
3: Ninja-ing. A very easy way you can be a huge help to your team is sneaking behind enemy lines and assassinating their healer. Genji can't stealthily teleport behind enemy lines like reaper, but he has the second quietest footsteps in the game [behind Zenyatta who floats], and can take routes not possible to any other hero thanks to wall climbing and Swift Strike.
For a starter Offense hero I recommend S:76 and Reaper. Both are very straightforward heroes, but S:76 is more team-oriented than Reaper.
Also, you'll probably like Mei. Her entire kit is being a dick