This paper first presents the design of a discrete-time general-structure controller, the order of which is determined by the process model order. Through the modified internal model control and dominant pole placement algorithms, the direct design is achievable by specifying one to three readily understandable parameters. Simple-structure controllers with user-specified orders can then be derived based on ingenious approximations of the general controller design. For example, a low-order proportional-integral-derivative and a high-order controller are obtainable in the same fashion. Furthermore, with a newly developed index for stability robustness, a methodology is proposed to optimize the controller designs of various structures. It is concluded that the resultant simple-structure controllers are vastly superior to the general-structure controller in almost all respects. The method is demonstrated with a wide range of process dynamics including time delay, right-half-plane and left-half-plane zeros, low- to high-order lags, integrators, and unstable poles.