I’m surprised by some of the answers here and what people feel is a soft smooth ride. Town Cars and Crown Vics really aren’t that smooth at all relative to most modern vehicles. As someone that used to work for BMW I can tell you to cross it off the list unless you like “sport comfort”, as they’re not tuned to soak bumps up so much as maintain composure over them.
It’s pretty much universally agreed on amongst automotive journalists, and most folks who’ve worked at dealers and who spend time in many cars, that the 1990-1999 Lexus LS400/LS430 is the gold standard for a soft smooth ride and whisper quiet interior. Not only that they’re some of the most reliable and durable cars ever crafted. If you want pre-2000 luxury there is nothing in the world (yes not even Rolls Royce back then) that compares to a Lexus LS for silence and smoothness.