This is something that I given a lot of thought too. My stock answer in the past has always been knowledge. While knowledge is a good answer and quite important, I think, there is a better answer. Your most important prep is the one that makes the difference between life and death.
Knowledge can be good, but if it is all book knowledge, you can die while you are trying to apply it. Book knowledge has to be converted from theory to a working knowledge or skill. A good example would be fire starting, it is great to have the knowledge that birch bark will make good tinder, but until you have tried it and made it work, you don’t have a skill.
You can have a backpack full of the finest survival products in the world and die while you are trying to figure them out. When you are working under adverse conditions, you don’t have time to figure things out. You need to know what to do.
There is an old saying beware of the man who only has one gun. He probably knows how to use it! I know people who have many firearms and are not really good with any of them. They go for numbers and gimmicks rather than skill. The same thing applies to your bug out bag or other preps.
On a camping trip, I was on a few years ago, there were two gentlemen with different tents. One was an expensive highly rated tent that was new in the wrapper. The other was a inexpensive Walmart tent that showed sign of lots of use. They both set up their tents and went to bed. During the night, it rained hard. The next morning the man who owned the Walmart tent was up and had a good night’s sleep. The man in the expensive tent had a bad night, it seems his tent leaked and he ended up cold and wet. The point of this story is the one man knew his gear the other didn’t.
Your gear does not have to be the best or the most expensive, it just has to work. So I guess there is no one most important prep. It is all-important, so practice with the preps you have and develop your skills. The preps you have and have practiced with are the ones that will save your life