Barring Zoe, of course, who is the strongest Star Guardian? Before the 2019 trailer was released a lot of people said that Jinx was the strongest but I can't seem to find any canonical evidence for that. Heck, is there even enough information to determine the strength of any guardian?
Obviously
I know nothing about the Star Guardian universe, but i do know that 2 is better than 1... and Jinx has 2 familiars: Kuro and Shiro.
So, i guess that she is the strongest.
or 
have my guess.
I think it was implied that Jinx has a lot of raw power, but it's tempered by her lack of discipline/desire for teamwork. Soraka also is mysterious; I think I recall there being something unique about her star guardian crystal.
I haven't done a lot of formal research.
Probably ahri.
I would say Syndra, Zoe, and maybe Janna. They all have similar power levels since they all came before the other Star Guardians. I'm just basing it off that the more experience a Star Guardian has, they are usually more knowledgeable about using their powers to highest potential. I would go as far as to say Ahri is also extremely powerful and Soraka might be too since she has that 8 pointed star and remembers what the First Star was like before.
Since they all draw their power from the same place, I'd actually say that they're all equally powerful. Even Zoe, particularly Zoe before her corruption.
My vote 
it's implied she rates among the highest in raw combat ability and experience.
Plus she has two spirits and plays the role of "the dragon" in Ahri's group.
When they first came out, jinx in gameplay was the strongest.
