In this article, the optimal model is determined for the family of models ‎$‎‎‎{E_r/M/1, rin N}‎$‎ with interarrival times with Erlang distribution‎‏ ‎and service times with exponential distribution‎. The method of choosing the optimal model is that first, a cost function is introduced, and then a new index is introduced according to the cost function and the ‎stationary‎ probability of the system called ‎‏‎‎$‎‎‎SER‎$‎.‎‏ A model with a larger ‎$‎SER‎$‎ index is ‎optimal.‎ Numerical analysis is also used to describe the method of determining optimal ‎model.