Moira is the best healer as she can easily outheal every other support, period.

[Savannah Dey Carden]

I've out healed lots of Mercys as Moira.

However I feel like Moira excels as a secondary healer, so she can pump out more dps, but that just how I like to play.

 

[Maxwell Searcy]

Technically Zenyatta can outheal all other supports in the game; it’s just unlikely. His ultimate lasts 6 seconds and can output heals at 1500HP/s. That’s 9000HP healing possible each time he ults.

There is nothing in the game that comes close to that kind of healing.

Edit: maths were wrong

 

[Jeroen Berghmans]

I just wanne see that happening. A moira do more healing than damage/kills.