For a Heal card to work, a few conditions need to be met:
I) You need to hit with the Heal card early
II) You need to then lose life lower than the cards max
III) You need to also not lose through KO
Both II and III are helped by a card with a high max, but also important is being in a clan that can reduce damage or use SOA or SOB to stay alive for the full four rounds. Cherry and Gretchen are my favorite healers for this reason. Sometimes you can toss eight or nine pills on Gretchen round one, and then just be defensive with DRing Roots and as long as they don't fury, win the fight off only that initial five damage! Also of note is Eddie, who with his clan's bonus has a power of 10 and often can hit round one. Because Eddie has a max over twelve, his Heal is almost always going to be live. Snowflake and Igor are both average. Ulawele will get you some heals, but he will tick it up slowly.
Some Heal cards aren't impressive to me. Ash is a misfit, because he's in a clan that likes to win as many rounds as possible (usually aiming for three round wins), and regains a lot of its life lost through +Life abilities. Eggman has similar issues where sometimes you're winning with a Jungo deck and you have 18 life. Lakit is in a rough position. He heals a lot, but his clan often has +Life and his minimum is so low, sometimes you'll just get KO'd trying to get the healing active. Gudvibz is just awful.