My wife, Carrie.
She does so many things better than me. Her ability to roll with whatever her players throw at her is astounding. Sometimes I think she completely runs her games off the cuff, but I know she has a world written up. Everything stays coherent and cohesive even when she is running on the fly.
She is able to provide her players with unique and interesting encounters and NPCs. She can also make us care about things we normally wouldn't...like the Mule. We once had a mule with us and as time went on we became more and more attached to the point where we decided the campaign would end if ever the Mule died.
She has so many other excellent DM qualities, (like being great at making up names, providing fair but challenging encounters, not letting us always win, etc) that she is the best DM I've ever had.