Smoothest riding car

Hard to say, you didn't specify a budget. But look at larger Luxury sedans or more expensive trucks. You'll want supportive comfy seats and a long wheelbase. A newer Ford F150 or larger, Volvo's have nice seats, something like a BMW 5 or 7 series, Audi A7, or any other luxury SUV.

Check out Lexus. German cars and SUV's are performance cars and tend to have a harsh ride. You need to spend a lot of seat time test driving cars.