straight six cylinder engines are generally smoother than any other engines in most cars. see below from wikipedia
There are four different forces and moments of vibration that can occur in an engine design: free forces of the first order, free forces of the second order, free moments of the first order, and free moments of the second order. The straight-6, flat-6, and V12 designs have none of these forces or moments of vibration, and hence are the naturally smoothest engine designs. (See the Bosch Automotive Handbook, Sixth Edition, pages 459-463 for details.)
Engines with particular balance advantages include:
Straight-6
Flat-6
Flat-12
V12