User blog comment:WolfKingDemon/Gas properties for orion/@comment-17969412-20160704015809

In terms of raw energy generation, pound-for-pound, antimatter obliterates practically every other material that ever exists - the only downside is that it's superhard to accumulate. Ortho-Hydrogen is better than the other potential, consistent sources of power, but it's nowhere NEAR what anti-matter can generate, and it's also pretty hard to accumulate ortho-hydrogen anyways (although not nearly as hard as the process to get anti-matter).

tl;dr: just use hydrogen ffs