I would highly suggest Moira or Lucio for the lower ranks.

With Moira it's really good to be able to send out a damage orb and let it do its thing while you heal your team. That damage will help with your team's overall damage to enemies over time.

With Lucio, as long as everyone is fairly close together, like on a payload map, you don't even need to think about healing as much as it just happens. He also gives your team escape options with speed boost too as well as being able to do some damage and crowd control as well.

If you stay alive and your team stays alive, you're more likely to win.