18650 is the best option for powering a handwarmer in my eyes. 10Wh in a small package, cheap and easy available.
What b42 said isn’t so dumb at all, the most energy gets transformed in heat in a flashlight, you can have different modes, it’s cheap and a small form factor and it can make light too…
I had once a small handwarmer powered with 2xAAs and I sent it back after testing it for a while, got not really warm and eat up batteries. Today I know that alkaline primaries are very bad for higher currents so that might be a reason why I was so upset especially compared to the cheap old style hand warmers with a coal stick inside them.
I have also one of the usbloads, it has a switch to adjust 1A or 2A load and it gets really hot…
Have you seen this: Electronic hand warmers? (Not a flashlight!)
Here is somewhere a thread about this: http://carbonheater.us
I guess a ideal handwarmer has a kind of temperature control, to prevent burning your hands and still warming if it is very cold…