From what I understand, with assorted chemical energy storage technologies you can get a (for our purposes) arbitrarily high power density... as long as you're

From what I understand, with assorted chemical energy storage technologies you can get a (for our purposes) arbitrarily high power density... as long as you're also willing to deal with arbitrarily high volatility. See stories about laptop batteries igniting for an example just with current batteries. Of course, if you're talking about scifi theoreticals, the thing to do would be to calculate the energy from mixing X/2 volume of matter with X/2 volume of antimatter (presumably with some amount of the total volume skimmed off the top for containment structure).