It would probably be the biggest discovery known to mankind if we found out how to counteract the "force" of gravity. It kind of seems impossible; given that GR is correct, gravity is geometric, and is a result of curved spacetime. Perhaps if we found out about the "dark energy" that's expanding the universe and acting against gravity, will we be able to create antigravity devices? Imagine the manipulation of gravity!! No more oil, no more pollution, etc. Space travel far and beyond, maybe even time travel.