I like the idea of a wood stove, because it's lower tech means you can burn whatever you have to for warmth. Maintenance is higher, but there are few dependencies in a SHTF situation.

My folks had an amazing modern wood stove at their old place in Hansville, WA. It produced great heat, yet it had a glass front door. The combustion air was delivered where it kept the glass clean, so you could watch the fire like a fireplace (which I love to do). It was all they needed to heat the whole house.