Let me fix it (also Opinion):

Top to bottom

Ana: Has the highest possible healing (that also combined with the biotic Grenade), the only thing that is bad about ana is that she doesn't have an automated self healing like the other supports and she (sometimes) has to share her healing bullets as a way to damage enemies if the situation is necessary, also she is required to reload to heal again. Ana wins the spot thx to her powerful support sniper kit.

Mercy (My main): She without doubt has a strong close range medic themed soldier (Like the Medic from TF2), her staff heals people with 70 HP/s (correct me if I'm wrong) and also damage boost players with 30%, she has the power to flip the whole battle with one powerful ultimate which is why she is the most targeted support of all, she has self Regen which helps her to survive and she can keep her healing on a single target forever unlike Ana who needs to reload every 10 shots, she takes a great spot behind Ana in the most ​healing.

Lucio and Zenyatta: I don't know what to say about this they seem mixed to me, Lucio can speed boost or heal multiple targets, and Zenyatta can place buffs (Harmony) and debuffs (Discord) over enemies and allies, I have seen a lot of people using them probably together, they really do work great because of the amount of buffs and assist they can give to the entire team.

Sombra in my opinion is in the bottom, while of course she can hack healthpacks to become better than ever, for my own experience I haven't seen a lot of players use my healthpacks as a source of healing.