By the Euclidean algorithm, the answer is the gcd of all orders of all non-abelian finite simple groups.  I believe that this is 4 (looking at the groups listed in Wikipedia, one can see that it is at most 4 since once can get down to 12 on the tables of low order groups, and the Suzuki group has order not divisible by 3).  My recollection is that a finite simple group actually cannot have cyclic 2-Sylow, and thus must have order divisible by 4.