The one that works when everything is wet.

A lighter fails in wind. Matches fail in rain. A magnesium block does neither, because you are not trying to light a flame — you are shaving magnesium into a pile and throwing sparks at it. Magnesium burns at around 5,400°F and does not care that the wood is damp.

How to actually use it

• Shave a coin-sized pile of magnesium onto dry tinder. More than you think.
• Shelter the pile — it blows away in a breath of wind.
• Scrape the flint rod hard and fast toward the pile.

Most people fail the first time because they shaved too little. Practise once at home rather than for the first time cold and wet in the dark.

No fuel to leak, nothing to go flat, nothing to expire. It lives in a glovebox for ten years and still works.