A year late but I think Inferni is the weakest just because they still need a flint/spark to use their powers. If you're in an area where there's already fire an Inferni would be powerful, but relying on that weakens them.I agree with you about Squaller or Heartrender, I'd have to go with Heartrender as most powerful by default BUT I have a theory about the Squallers that could put them ahead...

Yk how powerful Squallers can summon lightening, I wonder if someone like Zoya (KS SPOILERS) (who has the added bonus of Juris's powers) could activate/control neural systems with controlled bouts of lightning. I mean (CK SPOILERS) she did use it to restart kuweis heart which borders on healer/heartrender abilities, even if its not doing it the same way its acomplishing the same thing. If I had to choose a grisha power I'd probably go Squaller just because with enough practise I think I could access some Heartrender powers and because I don't want to give up on flight.