Are we limited by such mundane facets as reality? If not, the TARDIS, definitely, for what I hope would be obvious reasons (ability to go anywhere in time and space being a main one. Unlimited storage space being another - Sarah and I could move into a cheap two-up two-down and have all our possessions stored in the TARDIS, which could be parked in the back garden. Or even in one of the rooms in the house). (Or I could get the Chameleon Circuit working and the TARDIS could be the house!)
If reality is an issue, then some sort of Motorhome would be something I'd adore - I've always wanted one; just the idea of going away for a week or so and not having to bother about a hotel or B&B, just live out of a vehicle. It holds a great deal of appeal to me.