I don't like antimatter because it's the most volatile fuel possible. If power is ever interrupted for any reason for any amount of time, the entire mass explodes.

A slightly less insane fuel source is a micro black hole. Drag a tiny black hole behind your ship and drip-feed it any kind of mass you come across. You still get >90% mass-energy efficiency which is far beyond anything else we know of.

Besides, one of the big problems with antimatter is that it's a battery, not a fuel source. We must first collect the unimaginable amount of energy and then process it into antimatter one particle at a time. If you build a ton of factories around a star you can get meaningful production. But a black hole drive can suck up interstellar gas or any asteroids you come across. Matter is easy to get. Don't ask where the micro black hole comes from.