Versatility

[Kaden]

Solo Top
Mordekaiser
Nidalee
Lee Sin
Riven
Nasus
Gangplank
Wukong
Yorick
Garen
Xin
Udyr

AP Mid
Cassiopeia
Nidalee
Karthus
Kassadin
Swain
Ryze
Gragas
Jax

Jungle
Master Yi
Xin
Udyr
Lee Sin

Ranged AD
All of them. Seriously. If anyone think I'm lacking on one of the champions, let me know, because I'm not aware of it. This is my most competitive role and if there's something I can do to keep it that way, I'd like to know.

Support
Janna (best support in the game, btw)
Alistair
Sona
Swain (it's a thing, I swear)
Taric
Soraka

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In case it wasn't clear, Ranged AD and top bruiser are probably my best roles. So... yeah... there's that.

Really, I don't think there's a bad champion someone can learn for a given role. Just go look at the lineups from the previous MLG. People banning LeBlanc? That should say something about the balance of the game. Find a champion, for a given role, who's mechanics work for you and just master them.