I don't toss the term genius around very often, but if it ever applied to anyone it was Pieter Bruegel the Elder. What an eye! What a hand! What a spirit!
(The image above is a detail of his painting of childhood games.)
Bruegel puts so much into his paintings that he must have kept prodigious sketchbooks. Very few of his drawings have survived. Maybe they're stashed away in some attic in Antwerp.