What is the most important rule in Bud, Not Buddy?

I wish that I could give you a definitive answer, but that is just not possible.  Bud Caldwell doesn't rank his "Rules and Things to Have a Funner Life and Make a Better Liar Out of Yourself."  He numbers his rules for sure, but they are not ordered in any sort of hierarchy.  In fact, as the book progresses, it becomes clear that Bud just makes up a number to go along with whatever rule that he is about to give the reader.  That suspicion should start creeping into a reader's mind when Bud brings up rule number 328.  Really?  328?  That's a lot of rules that Bud supposedly has memorized.  But late in the story, Bud reminds readers about rule number 547.  The reader has never encountered that number before, but Bud says that he already told the reader about the "worry" rule.  

The first thing to worry about was Rules and Things number 547, or something, that was the one about when a adult tells you, "Don't worry."

You can see from the quote that Bud can't remember exactly which rule number the rule really is.  The "worry rule" that Bud is talking about was actually rule number 83.  

If an Adult Tells You Not to Worry, and You Weren't Worried Before, You Better Hurry Up and Start 'Cause You're Already Running Late.

As you can see, I would like to tell you that Bud has a specific number and rule in mind, but the numbers keep changing.  Bud also never says that a particular rule is more important than any other rule.  To Bud, each rule is very situational.  So the most important rule to Bud will be the rule that best applies to the situation that he is in currently.