A success of file transmission is determined by channel quality from wireless caching helpers in wireless caching networks. According to the caching strategy (what and how cache the files) of the caching helpers, the channel quality dynamically varies for given network realization, which can be termed as channel selection diversity via caching. This paper analytically identifies the channel selection diversity gain via caching placement and finds the optimal caching placement of stochastic wireless caching helpers with sophisticated considerations of wireless fading channel and arbitrary memory size. With stochastic geometry, we derive the optimal caching probabilities in closed form to maximize the average success probability of file transmission and propose an efficient algorithm to find the optimal solution. With simulations, we verify that the proposed optimal caching placement is superior to the conventional caching placement owing to the optimal control of the channel selection diversity gains for each file.