1v1 combat : Summer Sakura for sure. Her extreme healing and possibility to avoid death by sacrificing slug is simply amazing. Cloak Sakura used to be best 1v1 healer and if you are using either clone team (anything from wind main to edo Deidara) Cloak is better option. Really decent heal and unlike Summer, she has also offensive ability.
Summer Sakura was free, otherwise some 8k cps I think.
Cloak Sakura used to be 8k (and in one event for 600cps?).

For 3v3: Hokage Tsunade (but thats not f2p), so Young Adult Tsunade, healing up to 12 units.. no need to explain more... 8k cps for her (also you can get few frags from survival trial from time to time).

Btw, dont underestimate Xmas Karin. She is much more offensive healer, but can definitely cause trouble for your opponent.