If I'm going to be driving 3000 miles, among the common rental fleet, the Altima is hard to beat with it's 36mpg at 90mph. It just fits me well, darn comfy seats. With the note that it cannot be the base model. Must have the good power seats.
Among better cars, the Honda Accord Hybrid touring drives nicely. The better Camry Hybrid is not quite as good, seems noisier and cheaper feeling, but good enough.
Best for long trips among the rental fleet is a higher end Chrysler 300, preferably with the 5.7V8. It's big, thirsty, smooth and pleasant. Although the V6 can return pretty good MPG if driven carefully.
In fact, if I have passengers, the Chrysler V8 cars are best. Roomy, enough power to climb long hills and no drama.
One other weird car I really like is the Volvo S90 T8. It's a plug in hybrid that is fun to drive and flat out moves.