The results of a comparative numerical study aimed at assessing the mixing performance of planar zig-zag, curvilinear, and square-wave microchannels of square cross-sections is presented in the paper. To evaluate the mixing enhancement characteristics of each geometry, suitable mixing indices are computed at different axial locations of a single repetitive module of each microchannel when fed with two equal streams of fluid having the same thermophysical properties but different solute concentrations. To separate the effects of the geometry from those of molecular diffusion, entrance flow, and channel length, the mixing in straight microchannels of the same length is also evaluated for comparison. Reynolds numbers in the range from 5 to 150 are considered. The study is performed both with reference to a fixed Peclet number, equal to 2500 and obtained by scaling the diffusion coefficient when varying the Reynolds number, and to a fixed diffusion coefficient, yielding a constant Schmidt number of 16.$(6) over bar. Pressure drops are also calculated. All numerical simulations are carried out using an in-house finite element code for the solution of all model equations.