High temperature superconductors.

Specifically anything above commercial / household freezer (-18C); but if we could get to ~105C (above water boiling) it would change literally everything.

Electric motors become more efficient over a much greater RPM range.
Superconducting magnets become much easier to construct and run, this gives us a much better chance at fusion.
Transmission lines themselves are pretty efficient as it is, but all of the associated switchgear at the conversion points all gets really warm, this could be virtually eliminated.
The conductors on circuit boards, and potentially inside microchips. This reduces heat loading and thus makes all computing devices more efficient.
The conductors in batteries; enabling these to be smaller and thus increasing battery energy density.
Finally making super-capacitors actually viable as longer term energy storage.

There are so many aspects of life that would be impacted by this one breakthrough, that it is probably the most important thing that will happen this century (scientifically speaking). It would be almost as revolutionary as when electricity itself became widespread.