This can help you shed some light on the phenomenon, but overall a Mary Sue isn't a combinations of traits (i.e. flawlessness, being super powerful, etc.) but more how those traits are treated by the author and the characters in the story, how many there are, if they're justified within the story, etc. When they accumulate so much that they stop the story dead in its tracks, and prohibit developement, then you can say you have a Mary Sue became the process towards saying if a character is or isn't a Mary Sue needs to be holistic.
Now, from what we have seen so far, not one of the three main cast members can be considered Mary Sue yet, though if the writers continue to treat Iris the way they have been so far, she has the possiblity of becoming a Canon sue.
First of all, the lack of flaws. Now, both Iris and Dento have been shown to have flaws, but once again, how are those flaws treated by the writers? How do they relate to the story? In Dento's case, his flaw is arrogance, overconfidence and how he's very quick to judge other people. Now, on the two occasions that happened, he was either "punished" for it (losing to Ash in the gym battle) or was called out on it (Both Satoshi and Iris reacted negatively when Dento bet with Cabernet that should he lose, Ash would release all his pokémon). In short, they were treated as flaws. Whereas in Iris's case, she's shown to be very hypocritical and hypercritical, yet, it's not treated as a flaw, not shown to affect anybody's opinion of her and eveyone still wants to be her friend. Same for Doryuuzu, in her battle against Shaga, she wasn't able to read Doryuuzu's emotion correctly, yet Shaga and the village elder, far from being critical of Iris, they looked intrigued and impressed. The initial problem with Doryuuzu was also treated as something internal to Doryuuzu, not something brought on by a mistake that Iris made. So, Iris wasn't shown to be doing anything wrong in that situation, beared no resposibility in the problem, effectively stunting her developpement.
Same thing for being super powerful. On its own, it's a trait, nothing more, neither good nor bad. It all depends on how it's being treated in the story by the writers, whether or not it's justified within the universe. Dento beating Cabernet is justified in that he's studied longer than she has to be a sommelier, he's passed more exams (that are required to achieve ranks), he's been a trainer longer, etc. There's nothing that proves he's a sensational trainer, having lost to Ash and a wild Pendra, and seen struggling against the wild hitomoshi. Whereas with Iris, before most trainer even begin training pokémon, she had 99 OHKO wins against trainer who had been more experienced than her, had been training for a longer time, she was shown to beat evolved pokémon with her unevolved Mogurew. Cilan's abilities are shown to be within the range of the universe, ordinary if you will, while Iris's were shown to be extraordinary, uncommon, something that made her super special...
tl;dr any Mary Sue trait is just that, a character trait. It's how that trait is being treated in relation to other traits, how justified it is by elements of the story and whether or not they stop the story dead in its tracks. To answer the post, I'd say Ash is the most flawed, while Iris is the least flawed, not because the flaws aren't there, but because they aren't treated as such by the writers and aren't an hindrance, an obstacle for her.