There are many reasons why Steph is the best character in No Game No Life, but the biggest and main reason is that I am a Steph myself.
(Warning: This post contains dirty humour, for what is a Steph without embarrassing sexual incidents?)
It is commonly assumed by anime fans that the Harem Protagonist, the audience stand-in for socially inept Japanese males, has shit taste in women. Close analysis of any given harem anime reveals that a vastly superior Best Girl is ordinarily present within the harem framework. A question that presents itself is why Harem Protagonists do not simply bang the Best Girl when she clearly displays strong amorous desire for his dick. This systematic study of harem anime shows that Harem Protagonists are genetically encoded with bias towards girls with severe IQ deficit.