Three that are a little larger than mid-size, but they are quieter and smoother-riding than truly mid-size sedans:
Kia Cadenza (2014 only)
Chevrolet Impala (2014 only)
Chrysler 300 (2012 or newer, fairly efficient with V6 and 8-speed automatic)
You might also thoroughly test drive a 2011 or newer Hyundai Genesis to see if the ride feels good to you. The ride of this one varies by road type--very good on some, a little busy on others. Fuel efficiency is just okay.