Equally important, the both of them. Without a strong imagination, then this species would never be able to get anywhere during the course of time. Look at all of the major inventors during their time (Einstein included), they were both very imaginitive (if not somewhat crazy). Yet they used both a combination of imagination and knowledge to create something great or benefit humanity incredibly. Look at electricity. Look at mario for pete's sake! All of them had some claim to fame in the fact that they were all original. Some great movies were spawned from the sheer imagination of the creators. Look at almost all Pixar films (i.e toy story, Wall e, UP) each one took a tremendous amount of imagination and talent to make a reality.
However, without knowledge, you could do nothing with your imagination. Sure you could be that guy that plays with a lightsaber in front of a camera that is shortly posted on youtube, but that's not necessarily doing anything useful. While this guy was tremendously hilarious, he was probably imagining fighting god knows what or being the hero of his own world. Or just doing it for the lols, no one knows. But if Einstein did not know how to go about proving his ideas, if the team of pixar didn't have the skill, talent, or knowledge to create their movies then the ideas would just stay in their heads forever.
see where I'm coming from on this? It's an equilibrium of the two that brings about change and leads us to the ever winding road of life.