We own a Saris Bones rear rack which fits on nearly any vehicle, but as others have noted, that's a crappy solution when the weather sucks and/or if you're travelling very long distances. So when we rent vehicles nowadays the ability to get (at least) two bikes inside is a primary concern.
I've been pleasantly surprised at the amount of room in lots of 4-door sedans and SUVs we've rented -- and occasionally unpleasantly surprised at how some seemingly enormous vehicles have very little usable cargo space -- but my favorite for hauling bikes is still a mini-van, like a Dodge Grand Caravan. All the seats disappear completely (fold down under the floor) and then you can just wheel your bikes straight in without removing any wheels or seatposts. Bonus is that you can easily transport a half dozen bikes this way.