I'm very much a fan of the numbers 6 and 12 because of their many factors. If we counted in base-12 instead of base-10, and had a metric-like unit system based on multiples of 1728 (which is "1000" in base-12), we'd get both the simple notation and arithmetic of the metric system, together with very mathematically convenient units. 10 has the factors 2 and 5. 12 has the factors 2, 3, 4 and 6. It's far more useful as a numeric base.
@thor
I was going to mention that we have 10 fingers not 12, but we could teach children to count using their feet (that is with the feet themselves rather than with the toes) along with their fingers.
@tae_lesion If you read the gajillion articles on duodecimal that exist on the Internet, I'm sure you'll find one that explains how to solve those issues.