What a question indeed. I generally have a tendency to like music that is heavy or intense or otherwise invigorating. I also have a strong affinity for music with classical elements like Choirs and Orchestras, but also ones with electronic or modern elements intermingling properly. When it comes to lyrics, I have a distinct draw to them; if the music has lyrics, I really like them to mean something you have to pick up on, or tell a good story and paint the action or scene in my mind's eye.
Really difficult to pick any single song, but I do think I have one. It is missing a few things, but the elements generally fit all the categories I put up. Heavy and intense, elements from either classical or electronic (in this case, classical), tells a good story and paints it well.
Dark Chest of Wonders by Nightwish