High-efficiency micromixers are of critical importance in several industrial applications. Implementing chaotic advection to passively enhance fluid mixing is a well-established technique. The purpose of the present study is to compare different configurations of passive mixers. Six different microchannel designs, commonly used in the majority of mixing applications, were numerically evaluated in the present study. Both two-dimensional and three-dimensional serpentine-type microchannel designs were investigated. For each design, the effect of geometrical parameters was studied using a commercial computational fluid dynamics code. Results are presented in terms of mixing efficiency and pressure drop. It is revealed that increasing the flow path by providing compact fluid passages is not the only way to achieve better mixing; inducing a helical flow along the flow length has an additional benefit in improving the mixing performance. The results of this study can be used as a basis for further improving the design of micromixers, while they can also provide the engineers with guiding principles, not for selecting the optimum set of geometrical parameters for a specific microchannel design but for screening out the least efficient micromixer configurations.