I think the greatest discovery would be cold fusion. It would be amazing if they could find a way to achieve it cheeply and sustainably.
Having a source of power with such potential readily and safely available would change the way we do everything in the world. Third world countries could have reliable power. Space travel would no longer rely on fission, solar or fossil based fuels.
I believe it would bring on a new advent of civilization. It could be a discovery that is on the same level as fire.
The biggest problem would be letting it see the light of day. The gas and oil companies would have too much to lose. It is likely they would put the full weight of their resources behind keeping it from being adopted.
Much like what has been depicted in the movies, corporate lobbyists and lawyers would do everything they can to stifle that innovation.