If you want to master the secrets of being a healer you need to master lucio first. because you dont have to do anything to actually heal anyone just to be close, Lucio with a good Tank is a force of nature and alone he can wreck people in maps like Lijiang Tower,Illios and Nepal with his second Shot that push people
Tracer is hard to master because she doesn't deal enough damage, my friends call her the "softener" because she just go like a bee doing damage and letting the heavy hitters finish the job is not weird seeing pros using tracer and having like 50 kills because almost all of them are from support
I recently began to use Mei just for shit and giggles and wow, with her new buff i can kill EVERYONE, heal myself and keep the fight, i can kill freaking Genjis now with ease and her ultimate has a insane range that almost cover entire points, making captures easy, you can use it in point A of Kings row and nail a team kill if you know what you are doing