The greatest number of all time because it is:

the only even prime number
the only number where n + n = n x n (besides zero)
the atomic number of helium
the number of genders there are
the number of ears/eyes/cheeks/shoulders/arms/hands/ legs/feet you have
the number of fingers held up when making the peace sign
the number of useful things on a pencil
the number of doors on a refrigerator
the number of good rap songs there are
the number of d's in DnD
the number of different products you buy at the store involving paper rolled around a hollow cardboard cylinder
the number of legs on an unstable table
the number of times you need to change your underwear every year
a reasonable amount of cookies to eat
the number of words in PhD(despite 3 letters!)
the number of things that a spork is a fusion of
the number of KFC Buckets that you need to eat daily to become obese
the number of funny "Yo Momma" jokes
the number of bad actors who starred in "Transformers"
the number that Paris Hilton can count to

AND MUCH MUCH MORE!
2's greatness doesn't end here though...
look inside yourself to find every other reason why 2 is undoubtedly the best number of all time!!

the middle of 1 and 3.

The second number on a number line.

The total IQ of Presidents George H. W. Bush, and George W. Bush

the second number on a number line, also used in l33t speak as a question mark, because of its somewhat resemblance to the curve on the top of it. used much the same way as a "1" is as an exclamation point.

Two can be as bad as one
It's the loneliest number since the number one

The second number in the common numerical order.