What is the best tire replacement for the 2020 S560 Coupe? I am looking for the smoothest ride possible. Wheels are 19" stock OEM wheels

I doubt if the kind of tire will make any difference. Its the size wheel and tire that create the feel of the ride. I would suggest bridgestone potenza S04 or conti extreme contact or pirelli p zero

Continental all the way.

Michelin tires have never let me down in over fifty years of driving. Only The best.

My summer tires are Michelin Pilot Sport 4S. Excellent in terms of grip and road noise. A step change from the all seasons I was previously running.

if you put on regular tires and get a tire inflated and sealant kit from MB, your car will ride like a cloud. All runflats ride harsher than non run flat tires. I keep my tires MO stamped but only use Pirelli and I have 20” wheels. If you can afford to upgrade to 20”, do so. Brand should’ve make much difference but a standard tire over a runflats is a huge ride quality difference.