A day or two ago, I posted my nomination for the greatest mystery in all of physics: why is it that the "gravitational charge" (i.e. how strongly you couple to the gravitational field) is identically equal to your inertial mass (i.e. how strongly you resist being pushed around by any kind of force)?

Einstein's General Relativity is our modern theory of gravity, and it answers this question in an extremely satisfying and elegant manner. Specifically, gravity is not a force at all; it's the geometry of spacetime. All objects move through spacetime in as straight a line as they can; if they deviate from a straight line, it's simply because of the curvature of spacetime. Objects of different mass are moving through the same spacetime geometry, so they all will move in the same manner.

This, to me, is an amazingly simple and elegant solution to what seems to be a great conundrum. Yes, it's often convenient to talk about gravity as a force, but when we recognize it not as a force but just as the background of what's out there, the conundrum completely goes away. Quantum Mechanics is in many ways a more successful theory than GR, in that it has been much more widely tested, and its tests are more precise. But I find at least the "gravity is the curvature of spacetime" part of GR to be far more elegant and beautiful than quantum mechanics.