Thanos isn't and wasn't evil. All he wanted was to please Death, and when he realized he couldn't, he saved the universe, and redeemed himself. Carnage is more evil than Thanos.
Anyway, i'd have to say that the purest goes to the "god" of that respective comic boook company, and most evil goes to the demons or "devil"
I dunno about Superman - he can be pretty arrogant when he wants to.
There's loads of people who are "pure evil". Joker and Carnage spring to mind pretty quickly - but I'd say Joker overall.
I don't know who is the purest form of goodness. Spider-Man, Batman, Supes, Green Lantern, Silver Surfer are all classed as "heroes" but even they let rip and almost lose it sometimes.
I dunno - what about Starfire from the Teen Titans? She's probably the most innocent "hero" I can think of.
If we are talking about "innocence" as a form of ultimate good - then yeah I'd say Starfire - otherwise goody two shoes Supes gets it.