No one can rival hokage tsunade in healing large groups. However, her usability in one on one fights is very limited. Her sannin war counterpart is very versatile in that regard. Being able to heal twice and attack three times or heal and apply immunity to debuffs depending on the breakthroughs that are active. A close second would be summer sakura though she would need a very high HP in order to be an unstoppable healing tank.

Obviously Water main. Sakura Summer is awesome as well but not for group heal.