Everyone seems to agree that Mercy is the least effective healer because of her kit not being sufficient to keep up with the likes of Ana or Zenyatta being better since he can multitask while Mercy can only do one thing at a time. But I don't think any healer can be as exhaustive to fight against against. They're like cockroaches, nigh impossible to put down for good.
This is true until higher skill levels where you can stick a competent flanker on her.
And there is still the 5v6 problem.