From the Stan Lee panel at San Diego Comic Con 2010:
Lee was then asked who was the most talented person he worked with, to which he instantly replied, “Jack Kirby! And not just because Evanier is in the room!”
Lee continued to extol Kirby’s virtues. “He really was a writer with drawings and no one wrote better with him. And every drawing he did was exciting. I don’t even think Jack had to think about it, it just came natural. When Jack drew something, he started at the top of the page and worked his way down, it’s like it was all in his mind and he was tracing it onto the page. Jack was absolutely the most imaginative and talented guy I’ve met and I’ve met many talented people.”
Source: CBR
