The strongest healer without a doubt is the standard sakura 2 star version. Huge heal to entire team and removal of all debuffs, however the issue is that she is easy to counter, highly susceptible to well timed acupuncture etc.

Sakura ninja war is very strong, having used her on german and chinese servers. The snails provide essentially 5 healers on your team all capable of removing debuffs and healing. They also act as mini tanks with their relatively good health pool.

The snail spawn ability also costs 0 chakra and is prompt on her turn.