Grover has a great ult, great healing for the whole team, and amazing CC and utility and dmg for the team, making him clearly the best healer in the game currently.
Furia doesn't have great healing raw, but her burst healing + pocket + ult + dmg + CC make her very good currently.
Io has good survivability with Luna and very strong Heal per second and good dmg for the team.
Corvus is weird, he is good but not good at the same time. His pockets are the best in the game, but his late game viability when caut gets to 90% is pretty bad, as he is basically a heal bot with no utility for the team aside from his move speed bonus given to marked targets. Early game corvus is legitimately the best healer in the game, but lacks late game presence to be at the same tier as io and furia, but his healing and pockets are better than damba, ying, seris, and grohk, making him barely above them.
Grohk is C+ tier as a solo healer but B+ as an off healer, with another healer in the comp he does very well, like Grohk + Furia. Overall these healers are decent but there are better options available
Viable but seen to a lot as semi-throw picks, only play rei on Bazaar.