I think this is one of the better lists that I've seen. I agree that corki is powerful, I'm just having some trouble with him. i tried to find a way to pm you, but I don't see that option. Is there a build you recommend? I usually go sapphire crystal>boots>black cleaver>mobility>frozen mallet. When I have to carry I usually alternate between corki and tristana. Corki's abilities are fantastic, but tristana seems to have a higher sustainable dps. Any thoughts?

I'd lower Amumu a tier, I know he can be really great, but that's often when the rest of the team has big AoE's.
On another note, I think the tier lists should be categorized to main roles.

Good list, it definitely depends on the enemy make-up. Janna probably doesnt belong in the worthless category, her laning stage is pretty good, especially if you go dps. Soraka and Zilean to a lesser extent are worthless because of the ease of getting executioner's calling, which should probably be nerfed (the lifesteal portion of it). Mundo should be in the worthless category due to executioner's calling and the fact that he has no late game whatsoever. Janna at least has aoe knockups and bonuses to the team.
The only way Amumu should be that high is if he is taken in a very aoe centric team, otherwise he isn't that good.

The very worst champions right now are warwick and eve, especially eve, oracles shuts her down too hard, maybe she needs a toggleable ability that makes her phase shift, making her immune to all damage and undetectable for a short time (can only activate in stealth)