s-
Jenos is one of the easiest and also one of the most effective healers. He has the capability to heal 3-4 people at a time and with Luminary they all can have dmg buffs. He can also carry solo matches with Binary Star with good accuracy since with this legendary he has more primary fire dps than Lian.
a+
Both Mal and Furia have the most healing per second in the game. Mal with gourd and with his right click has always been a good healer in skilled hands. And with the change of his legendary to now double the healing of gourd he can focus right click healing to other players and let tanks heal with it. With Furia her q with solar blessing has the biggest healing ability of the game which if played well lets her focus healing others and doing dmg.
a
Both Io and Seris are not bad champions but the problem with them is that there are champions that do their job better. The strenghts that they have is that Io can keep stunning tanks in the point with Luna and Seris has self healing with a good escape (if not countered by illuminate) and good single target healing.
a-
Ying can go up a tier without problem, but in order for that she needs skilled hands. Both positioning of her illutions and use of her q need a lot of time practicing in order to master.
b
Grohk is a weird one. He has the potential to go to A tier but there is not much play of him. His healing with totem and the legendary Spirit's Domain is no joke. The biggest problem I see is his escape. If a flanks catches him, his Ghost Walk is not going to help him much. But he also can be played as a dmg/healer hybrid, wich I think is the role that unleashes his full pottential. And Pip is much of the same. His healing with Combat Medic is really good, specially with heavy point-oriented comps. And he can get q resets with the right build.
c
If you have to play main healer, avoid him. I think he has one of the worst kits in the game for a main healer. He can work as a sniper killer with his big long range dmg and sustain, but his dmg is lackluster when trying to main heal. His passive forces you to stay beside the tanks,, has no tanking capabilities, having an ult that has a big dropoff thanks to caut late game and a healing ability with long cooldown he is just not worth it.