If I could only ever own one car, I would own a BMW M5 (E60). It is practically enough to seat 5 total, carry lugguage, comfortable enough to drive a whole day in, and fast enough of a car.
If I was to cheat and sell off the car for more cars, I would get a Bugatti Royale, sell it off and use that to buy myself, uhhhh... a lot of cars.