[Exit Strategy v1]

I know about fossil fuels and such but what are the top3 most efficient fuels out there?

[hmx9123]

I believe the two most efficient fuels by weight (i.e., calories/gram that they produce) are hydrogen and diborane, in that order.  That is just what I have heard, don't know for sure.  Depends a lot on what you want to do, though, if you want specific impulse, smooth burning, long sustained burning, etc.

[Exit Strategy v1]

well, I figured it out but hydrogen was one of them.