Depends what you mean by strongest player.
Similarly, if you said strongest player;
-Anima is technically in the running for that, due to her wide variety of summons
-Ashley has a one-shot katana
-Samson counters everyone short of Will, Albrecht, Izzy and Dragon!Stella
-Tobias is, well TOBIAS.
How about just limiting personae even more and replacing persona users with summoners who can have one of each (persona, id, stand, asset, Lesser Key of Solomon, meta-entity, etc)? Then reclassify Anima as one, so that one of her summons represents her fiscal future, another her facade, third her desire to [REDACTED] and die (not in any given order), a fourth named after a contemporary rapper/musician/song/band (or establish that there is a contemporary rapper/musician/song/band with the same name as one of her summons), Andras would obviously be the Lesser Key of Solomon summon and designate another as a meta-entity. Thus Kyle's sole summon at present is the Persona Merlin, Yuri's sole summon is the Meta-entity (seeing as it seems to be a reference to something) Emiya and they'd be able to get more.
If it wasn't for the fact that it would ruin the whole concept of Anima in the first place and unbalance the entire thing again, I would.
Also, this game is hardly about balance
I'd say some random perks for people would spice the thing up, make them all a bit more distinct mechanicaly. I doubt it would break out into class wars.
It nearly did anyway. I'm not nerfing Personas, but I AM willing to allow random perks on level up for anyone; granted, magic people would just get pure passives as they have a wider range of ability use than Personas and their skillset.