Coupes don't generally have excellent ride quality because they are usually fitted with lowered suspension and 45, 40, 35 or 30 section tyres. One of the best was the Peugeot 407 coupe, but the V6 petrol engine in that was not much cop. If you can get the new Mercedes C Class coupe fitted with air suspension (a £900 option on the saloon) that might be okay.