Mixing of fluids has numerous applications in laboratory processes, and several different designs have been proposed in the literature to achieve this objective at the microscale. In this work, the mixing efficiency for various spiral microchannels is computed numerically. We also propose a new geometry, a Golden ratio spiral microchannel, which is found to have a higher mixing efficiency as compared to four other types of spiral designs (logarithmic, Archimedean, hyperbolic, Euler) tested here. A Golden ratio spiral microchannel involves a stepwise varying radius of curvature. The larger mixing efficiency of Golden ratio spiral microchannel is owing to pseudo turbulence, created through a superposition of different vortex formations. The mixing efficiency over a large range of Reynolds number (5 <= Re <= 100) is studied to generalize the findings and the results apply till at least Re = 1000. The present results will be useful in the design of industrial micromixers for various applications.