Favorite questions (that I did not answer but I find fascinating):

Proving the sum of a product is equal to pi
Double integral of a Gamma Function
An interesting summation
An interesting number theory answer
The convergence of a Zeta function product series

http://math.stackexchange.com/questions/1154807/cauchy-euler-equidimensional-de-x3y-leftx-right3x2y-leftx-right5xy http://math.stackexchange.com/a/1213835/167604