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.
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.
Both are necessary....We need knowledge to learn what already exists.....We need imagination to create and invent for the future, therefore adding to our knowledge.
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