Benzene, n-hexane and ethyne in the decreasing order of add strength are ethyne > benzene > n- hexane, i.e., 
Addic character is based upon the presence of s-character of C—H bond. Due to sphybdridization in ethyne, it is 50%, in benzene it is 33% due to sphybridization while in n-hexane, it is minimum, i.e., 25% due to sp3 hybridization. More the s-character of C—H bond, more the acidic character.