That's an easy one. Just about every prepper list I've ever seen says "Coleman fuel - impossible to have too many."
These cost about $11 for four cylinders, and they last a surprisingly long time. I've got 80 of them in my basement, only taking up two shelves. Dunham's has the best price I've found. You can buy single or double burners to go with them, and even a heater attachment.
Also buy a couple of extra tanks for your gas BBQ grill. I have five of those.