I'm gonna go a little outside of everyone else's suggestions. '13 Taurus, Flex or MKX w/ MyFord Touch.
My cousin has an Edge with it. It's by far the best set of in-car electronics available. On a long trip, you're going to want easy access to your music, nav, the ability to grab some quick info off of the internet, etc. I chose the 3 cars that I did because they're large and comfortable. If going cross-country, that's a factor that outweighs the car riding on rails. The Taurus will still be fun to drive, not as much as an M5 or something, but a good blend of comfort, technology and fun.
If I had to drive from Maine to San Diego today, I'd just take my Outback. It's not the most spirited car, but it's really roomy, comfortable, and I know that I can take it off the paved roads for some fun.