Tricky question! A limo would be nice for going from city-city in about 5-8 hours, a train would be nice for going from state-state in around 24-70 hours, and a powered boat would be great for going outside the continent. But if I had to pick just one...
I would pick a Limousine (H-2 style!)
Would you rather shrink to 1cm tall and live for the rest of your life like that, or would you grow to 30 ft tall and live in that size?