In case someone hasn't said it, just pick MK. He's basically your brain in character form, and you're really only punished by dumb mistakes rather than character flaws.
Not all brains work like Metaknight players'. I'd imagine DEHF or someone of that sort to actually be worse at MK than their main in all eventuality simply because of the defensive/offensive/whatnot playstyle they have for approaching the game.
But I do agree, use what technique you feel is strongest in approaching the game and find the character that best applies to that perspective. I'd imagine Overswarm uses MK because he thinks of a controlling physical style with planking as a defense as the best way he plays the game.
Overswarm, shut up. You lost to a Sonic
Flaming on a guy because he loses while an entire community is watching not only in person but LIVE and mentally going "LOSE LOSE LOSE" is not very fair. I doubt you'd beat him with Sonic in an environment without an audience pressure on either of you. That's just my honest opinion.
Dark Peach, ignore what character you're best with currently. Think what character's playstyle appeals to you most. What's your favorite aerial, what's your favorite position relative to an opponent, what's your playstyle exactly (ask a friend about that one imo). Now determine the character that's right for you and take it a notch up for the hell of seeing what comes of it. Then go back to peach and decide whether you still want to use Peach or to stick with the newcomer to see what happens with it.
Edit: I remember meeting Bigfoot (California DK main that's ranked) not so long ago and he was using Snake in tournament. ONLY Snake. I was like "Why aren't you DK? You're ranked with him!". Well think about it, it's just like Ally trying to pick up MK. He wants to find the character that he's strongest with in the end. The strongest potential. When you exhaust your character's worth, you either are doing something wrong that'll be hard to find (and probably will most easily be found by taking a break and evaluating the character).
Or you need to switch mains.