A good mentoree.
Ok, that's simplistic. But really it doesn't matter how good the message is or how informed, passionate or helpful the mentor is, if the person on the other side of the relationship isn't interested in listening or discussing or trying new things.
An active, passionate, engaged mentoree will make even an average mentor (are any mentors average?) better by the questions asked, the discoveries made and the new territories explored.
We should probably discuss what qualities a good mentor may have... feel free to give me your top 5!