The holy grail of science
is a room temperature superconductor.
(funkenstein Born from an egg on a mountain top, Fri 26 Sep 2008, 21:34, Reply)

Room temperature is a bit much to ask...
... but engineering would be satisfied with one where the energy cost of cooling the conductor was less than the resistive losses :)
(douchebarge, Sat 27 Sep 2008, 6:21, Reply)