There are 2 questions when looking at your champ pool.
1 Do you love playing the champ.
2 Does it have HARD CARRY potential.
Sadly I think things like CC tanks, and healer supports, rely on their team too much for escaping bronze in a timely fashion.
It does not have to have the same snowball potential as say Yasuo, but more than say Braum.
Once you have chosen a champion, play it a LOT, until you master the subtle things.
Knowing exactly how to last hit for example. Every champ auto is a tiny bit different.
Using your skills to farm perfectly. Knowing exactly "If I auto this minion, then Q here, then auto that one, then E there, ill get all the CS!"
Knowing how to beat counter-picks, or at least just miss a few CS, rather than feed.
Your question is kinda like asking "What musical instrument will make my kid the best musician?"
There are probably some bad choices, like the tambourine lol, but for the most part, its about what instrument the kid enjoys, and practice, practice, practice.
Last season I escaped bronze mostly playing Illaoi, not so much because Illaoi is "the key" but because I love her, and have a ton of practice.