This is so difficult to answer because it is almost all subjective rather than objective.
A bit like asking what music is best. 'Ride' and 'best' are both remarkably 'woolly' terms in engineering terms.
I think the answer to the question is a Citroen CX estate from the early 80's. This one was a car that Martin Brundle couldn't sell off the forecourt and became part of his F3 budget contribution in '83. I inherited it as a company car. The pure ride, on long travel suspension and tall sidewall tyres was wonderful.
However, if you showed it even a photo of a corner at speed, it got very upset. Small yacht in a swell comes to mind.
So we have to have a compromise, and some of the current computer controlled air systems get very close on ride quality and are light years ahead in retaining control the car in extreme situations. The infamous 'Elk test' did ride quality no favours as car safety climbed higher up the priorities list.
There have been a number of really good shouts on here already for 'best riding' cars and I remember my Mercedes 124 being a really good compromise of it's time. Interestingly, the trend in modern compromise of ride and handling was illustrated by my change from a 2008 ML (20'" wheels and Airmatic) and the 2017 GLE (20" wheels, Airmatic) The pure ride quality of the ML was better, but the GLE was more composed in cornering and rolled much less.
My current A8 on air and 20" is a really good compromise. The 20" forged wheel/tyres are actually lighter than the standard 19" and this contributes.
I car vouch personally that the German roads are 'better engineered' than British ones. After setting a lap record for a road car at Nurburgring with an XJ220, we assessed the ride quality on roads outside the circuit and found it fine. Driving the same settings back on A and B roads back in Britain and it felt much too stiff.I spent the next 6 weeks revisiting the spring and damper rates!