I've driven my old 1997 Monte Carlo across the country at least 3 times, I drove a 1998 Buick Riviera across the country and halfway back once, and I've done the same with my new 2006 Monte Carlo, although to be fair I didn't make it all the way to the coast on that trip, only got as far west as Vegas.
Of the three I'd choose the 1997 Monte in a heartbeat. It was easily the most comfortable and the miles just rolled past the easiest. Large enough to hold everything you need but not excessively large. The Riviera is probably a close second, it mainly loses out because it didn't have XM installed, it was a little cumbersome to drive and I'm really not a fan of the power steering which has WAY too much assist. The biggest problem with the Riviera is it needs premium fuel and even with the seat all the way down and back, if I sat upright my head was still against the roof. My new 2006 Monte is probably my third choice. It's absolutely wonderful for short trips and medium length drives, but the seats are just too hard (and to think of all the cars I test drove this had the most comfortable seats!) for extended driving. It does have as much, if not more power than the Riviera however, which is great for hill climbing and passing; while simultaneously only requiring regular fuel *and* getting better fuel economy than my 1997 Monte which had a considerably smaller engine. Really my biggest gripe is the hard seats, I don't understand why new cars have to have such hard seats. Give me a 1970's Buick any day, they can shove this sporty ----; I don't need to fly around corners and not have my butt slide. I'd rather have a soft suspension and big cushy seats. Even the Buick Lucerne rental I drove for a week had seats that were just way too hard.
Now, if I could pick ANY car, I'd gladly make the trip in that perfectly restored 1971 Buick Riviera boattail. What an awesome car for road-trips and cruising around the Route 66 states. If I had to pick a newer car I'd probably go with the Cadillac STS that Matt_S mentioned. I haven't had a chance to drive a car with adaptive cruise control yet, but I'd imagine it's a pretty neat feature. A Mercedes S-class might not be too shabby either. No way in ---- I'd want to do it in any kind of truck or SUV.