Collision side-channel attacks are effective attacks against cryptographic implementations, however, optimality and efficiency of collision side-channel attacks is an open question. In this paper, we show that collision side-channel attacks can be derived using maximum likelihood principle when the distribution of the values of the leakage function is known. This allows us to exhibit the optimal collision side-channel attack and its efficient computation. Finally, we can compute an upper bound for the success rate of the optimal post-processing strategy, and we show that our method and the optimal strategy have success rates close to each other. Attackers can benefit from our method as we present an efficient collision side-channel attack. Evaluators can benefit from our method as we present a tight upper bound for the success rate of the optimal strategy.