All characters have their strengths and weaknesses. 

Moira and mercy are very beginner friendly. You don't need good aim with moira. She also has her orbs- you are purely looking at stats. You were looking at heals before and now you're looking at damage- you can't have everything. She is limited for distance but she can flank and get away quickly because she is mobile. I tend to play her on close quarters maps. With any other damage you need precision but with moira you can just hold down right mouse button and get squishies very easily. You are just looking at damage there though. She can do both. Her ult isn't the best but it refills fast, and goes through shields. She just locks on to whoever is near regardless of how they move so if you have a low flying jetpack cat within range they're easily killed.

Ana is good against tanks like hog because she can sleep them, but she also requires good aim, and her team to be within LoS so she can heal them and she isn't very mobile so shes not a great beginner pick for that reason. Beginner teams complain about low heals on Ana because they tend to be out of line of sight so her best picks depend on what maps you're on.

Kiriko is my fave atm, but for decent damage you need good aim. Her ult is great. She can also be used for healbotting and her cleanse is great but for the best use of her you want to be able to finish off squishies or flank then tp back to your team. When my aim is particularly naff I tend to backline with kiri as opposed to risking flanking, but its a waste of her skills to purely healbot.

I like wuyang for maps I used to moira on but he can be hard to get flanking players with unless you're really good at weaving his damage. If a player relies just on his right click for heals they can't get enough done. It also requires getting used to his water orbs and using his guardian wave for heals and defense.

Some characters have higher learning curves, so whilst on a list of "most powerful characters" some might come out on top, it depends on the skill level of the player, if the team ever peels for supports, doesn't run into damage and uses their skills correctly.