I think it depends on what you mean by "most important". Seems obvious, no doubt, but do you mean "the one I use most often" or "the one I would least want to be without in a survival situation"?

The tool I use most often is my SAK Classic, mainly because it's (almost) always on my key chain. However, if I were actually in a survival chain, I would probably trade every knife I have ever owned for a way to start a fire.

As Chris Kavanaugh says, there is no "wonder tool" that will be the ideal survival tool in all situations. Lost in the desert in summer, you could probably survive without fire but not without water. In a Canadian forest in winter, you could survive longer without water than without fire. given a cup, you could melt snow for water. On a mountain in the Andes, a spark-lite or Blastmatch would be just so much extra weight without fuel for your stove, etc.

There really is no definitive answer, IMO, because the "most important" tool depends so heavily on the situation.