“Comfort” is an entirely subjective quality, and no two people will judge it in exactly the same way.
Here are some thoughts to consider:
For many people, “comfort” includes the ease of getting into and out of a vehicle.
A small crossover/SUV (like a Rav-4, or CR-V, or Forester) excels in this area, as the seats are–literally–butt height, so you don’t need to lower your body when getting in, and–even more importantly–you don’t have to strain your back or legs when getting out of the vehicle. You just literally slide in and out of one of these small crossover/SUVs.
Seat comfort is totally subjective, based on each person’s anatomy and his/her particular physical problems. A seat that might be comfortable for one person could be absolute torture for somebody else, and nobody can possibly tell you which car seats will be comfortable for YOU. Only by sitting in a vehicle for an extended period of time will you know how its seats will feel to you on a long trip.
A vehicle that has electrically-adjustable seats will allow you to adjust the seat’s height, and its inclination, and the angle of the seat back with much greater flexibility than a vehicle not equipped with electric seats. Rental companies frequently use the base-level models that are not equipped with electrically-adjustable seats, so this is something that you really need to check with the rental company before making your decision.
An adjustable lumbar support is a must for many people, and–while most cars nowadays have an adjustable lumbar support–some cars lack this feature.
The bottom line is that YOU need to sit in the exact same model of each car that you are thinking of renting, and you really need to sit in them/drive them for an extended period of time in order to know what works for you in terms of comfort. This may involve visiting car showrooms and pretending that you are shopping for a car. This will involve quite an investment of time on your part, but that investment of time will pay-off handsomely in regard to your comfort on days when you are behind the wheel for 8-12 hours.