10.

Ten is such a nice even number. It equals the number of fingers on both hands. Modern man looked for a way to keep track and that was the first thing he saw.  What could be more obvious than building a counting scheme around 10?  It's amazing how pervasive the number is and how much impact it has on our daily lives.

Everything we do is impacted by our base 10 system of numbering.  The decimal system with all of it's inherent logic results from our system of 10s.  The metric system of measures uses it to great effect.  Our monetary system relies on this concept. We even use the number "10"  to score the perfect result - whether we're talking about figure skating or the ideal physique. 

What if we used a system based on the number of fingers that a Tyrannosaurus has?  That would be 2 fingers on each hand for a total of 4.  So your $5 bill would be "11" in a base 4 system.  I don't even want to think about the number of slots you would need in the cash register drawer to handle the change.