The venerable Vivitar 283 and almost as old 285 get my vote. I have an extra battery holder for it, a couple of wood "dummy" batteries in one of them with about 30 inches of regular lamp cord connecting it to a microphone plug. A small cut-out in the sliding door lets the cord come through. Years ago I bought a 4 D cell battery tray from Radio Shack and epoxied a female microphone socket to the side. It fits in a belt pouch.
D cells cost about twice as much as AA cells but have EIGHT TIME the power. What kills batteries fast is the heat resulting from rapid discharge. It's about impossible to fire a 283 even on full power fast enough to heat up the D cells.
I mostly hold the Leica in my right hand, the flash in my left, and for direct flash I try to keep the light directly over the lens to minimize side shadow. I also keep half a dozen business cards under a rubber band on top of the flash head. Want to do ceiling bounce? Pull the top card part way out and bend it forward a bit to kick some fill light towards the subject. If the subject wants a card there's no fiddling in your pocket.