I'm really surprised you have so many of the bot laners so low. Cassiopeia, Brand, Soraka, Anivia, Cho'Gath, for some reason you rate Malzahar, Karthus, Yorick and Heimer over all of them; you also have none in Tier 1 which I find peculiar; are you saying there's no best pick for bottom? In my experience, it's very hard for any of the bots to lane against a Cassiopeia, for instance.
As for "Why Soraka?" Well, because she has a massive Magic Resistance Reduction on her Q; allows her to deal stupid amounts of damage in longer engagements with no AP (which in turn allows her to build tanky to fight the longer engagements). Her Q is also one of the best pushing skills in the game, costing very little and doing pretty good damage over time.
I understand this list caters to the popular sentiments of the forums but does that truly make it accurate?