Loki is hands down the best healer in the game

I know everyone considers Luna the best because of her Ult, but Lokis shift is basically Lunas Ult. (Has a few more weakness given its not an Ult but...). Loki can literally use Runes to group sustain, when that ends use Ult to copy Luna. Use Luna Ult, then when that ends cool down for more Rune healing is up.

His AoE heals are way easier to hit then Lunas, then add in his clones to get serious amounts of healing/DPS.

Loki can very easily apply pressure to team fights, both through healing and DPS. The health Sustain from Loki is just insane. Dropping Rune healing at the right moments completely flips fights.

The Loki Hela teamup is so nice. Hela being able to rez both your healer (Loki) and your tank (Thor) just by playing her role.

.....and then there is his Ultimate. Literally being able to pick any character from either side to copy, and have their ult. Allows you to run 2 of any character. (Double punishers is always fun). Stealing their spidey and dropping an ult on the team, or just taking their Magick and running through their team. (Sometimes i'll just swap out loki for Magick for a life, catch them off guard, and revert back) or the classic hela ult. Playing loki well really comes down to how well you can play the rest of the roster, but thats not even anything to do with healing. the rest of his kit handles his healing better then any other supports entire kits do.

Mantis has the CC, and Adam can rez the team, but loki plays best when he doesnt need to carry heals for the team so he can Ult off without leaving the team dry.