Austin Princess with Hydrogas suspension...short of that any None sports model Toyota Camry or Lexus GS.
Anything American will probably be good for you. As long as you don't get seasick. If you are in doubt, go for the biggest car you can find. Bigger cars are in general smoother than small cars. Because the American car industry stopped in the 60's when talking about technology. The mustang still have a live rear axle! And don't get me started on engines...
*Googles* Wow, finally they've gone for an OHC design with 4 valves per cylinder. That'd impress me a lot more if Rolls Royce wasn't making OHC engines with Aluminium blocks and 4 valves per cylinder back in the 1930's >_> Mind you, cars are just way behind anyway
I love US rentals, I can rent twice as much car for half the price at age 21 Get an SUV with big squishy rear axle, and dump a load of heavy stuff in the back, then sit them in the back. Or a minivan.
take a look at what old people drive around: the lincoln town car. electronic air suspension. heavy. huge, fluffy seats. these cars are so freaking smooth riding its retarded. plus most (if not all) came with leather, heated seats, gobs of leg room. trust me this is one of the most comfortable cars to ride in. just dont expect to win any races that involve corners with one.