Antimatter would be the best possibility (i know the joke is done to death), other than naquada of course.
As it is basically matter annihilating itself into pure energy, so yes if we can utilise the most part of it, it should be the most efficient form of
energy.
There is something i heard that they are experimenting in doing it the opposite way, turning energy into matter 
BUT like has been said before, the only problem is creating antimatter. We will have to find a economical method before we can use antimatter.