What's the best number?

"The best number is 73.
Why?
73 is the 21st prime number.
Its mirror (37) is the 12th and its mirror (21) is the product of multiplying 7 and 3.
In binary, 73 is a palindrome, 1001001 which backwards is 1001001." 

This passage appeared in the 73rd episode in the entire Big Bang Theory series.