Neither usage is appropriate. Without context, I have no idea what this "most important" thing is (perhaps, as the statement implies, simply because I'm a human being).
It would be perfectly reasonable to end the sentence at the word "important". Semantically (but not stylistically) it could reasonably end with "to human beings" as far as I'm concerned, but someone else might say it should end with "to God's plan", "to world peace" or just about anything the writer thinks is important, but which he thinks others don't "get".
EDIT: Thanks to @RegDwight for pointing out an obvious interpretation of what's most important that I simply failed to spot. If what a person (i.e. - people in general) fails to understand is what's important to another person or to other people, the form should be...
Human beings often fail to understand what's most important to others.