Who should I grind?

I have most champions. I honestly tend to just pick someone at random when I play games just for fun and in general my goal is just to be effective with about any champion. I do however like the idea of picking a champion and playing them consistently for a while to see how much I am able to improve with them. I am curious who you think I should pick.

It might be easier to tell you who I don't have. Unfortunately, I can't put it up to a vote because Boards wont allow for that many options.

I HAVE EVERYONE EXCEPT Jinx Draven Darius Nautilus Fizz Lee Sin Renekton Cassiopeia Swain Galio Gragas Kog'Maw Olaf Pantheon Sona Urgot

Again, I have everyone else and I am looking for who I should focus on for a while.