Very good question. The artist in me wants to say imagination - but I think that as much as we all love children's drawings, and naïvity, a healthy level of knowledge is also required. The best artists, scientists, writers etc. had a lot of both. <br />
Einstein argued that imagination was more important, but I do feel that he was highlighting it's largely ignored significance in both maths and science.
I cant imagine or know what it would be like to lack one of them.
i'd take imagination,i mean whats the point of having knowledge and doing nothing with it