That is correct.
But I would take the bad faith out of that statement and say instead that it's the player who pays attention, who engages with the scene, and who makes solid decisions based on that in pursuit of automatic success over rolling that fickle d20 that is the most powerful character, regardless of class or build.
Similar to "how do I kill my players" threads, you're confusing character and player. Otherwise I'd say the most powerful character is the one being run as a DMPC...
Since nobody has mentioned it at all, I'm going to give a shout out to the PALADIN as one of the more OP characters in and out of combat. They can literally do everything well.
Since they are not real, characters cannot pay attention, engage with the scene, or make solid decisions. It's the player who does that. So I would say any player who does those things controls the most powerful character at the table.
And if two players do those things? Or if all players do those things? Or if a player does those things but feels hamstrung by a character? As a somewhat strawman argument, one could easily create a character that is unable to engage scenes, and one can certainly create characters that engage scenes, so there's at least a binary measure. Is it truly impossible that a character might exist that engages scenes better than other characters?
So...
The best character is one that best allows a player to engage with the group, with scenes, and best allows all involved to create powerful scenes. Characters with some backstory, but enough gaps the DM can fill in. Characters with personality, both positive and negative aspects but no absolutely crippling traits ("afraid of monsters" would be fine, even though disadvantageous. "catatonic when encounters monsters" seems like it would be incredibly limiting).
But above all, I think the best character is one that the entire group can enjoy interacting with.
I could make an argument that the most powerful character might end up being the opposite of that.
I do agree that the best character is one that the entire group enjoys. It still takes an effective player to make the character do that though. Effective players matter more than mechanically-capable characters in a game where the DM decides on success or failure before engaging the mechanics.