Zen is my carry healer pick if the team can't kill anything because discord actually lets the bads dps something down without a great deal of focus.
Yeah, Zenyetta is my best hero to rank up. You fill a healer role which is always good, all you need is 1 more person healing. You provide good DPS on your own. Discord is just awesome, at my rank rather than putting my orb on someone and calling it so people focus it(not happening), I just put discord on whatever they are fighting. Exception being when there is a Pharah, I tend to leave it on her to help the DPS take her down. Finally, transcendence is one of the best ultimate to save your group.
I've switched to Moira, right now, I'm essentially about the same win rate, 1-2% lower than with Zenyetta. She's very similar, you provide healing, dps...where she's different is with her ultimate, it's not as impactful as Zen's transcendence, but you get it maybe twice as often so it impacts a lot of fight but not as dramatically. The only reason I favor her right now is that she's new, and I enjoy her playstyle more, mostly her mobility and self sustain. Overall though she isn't really better, just more fun in my opinion.
The most frustrating thing as healer main to rank up, is that you can feel a bit helpless at times when the DPS are not killing stuff, no matter how well you heal, if people are not killing you'll lose. You can win more easily if your team kills with some average/bad healing.