If I ever become a billionaire or hundred-millionaire, here are some residences I would consider:
Updown Court - Windlesham, Surrey, UK
Updown Court - Windlesham - Surrey - The UK's most expensive and luxurious private residence
Hearst House - Beverly Hills, CA (aka "Horsehead Manor"; this is producer Jack Woltz's palatial estate from The Godfather)
Neverland Ranch - Los Olivos, CA (I'd have to exorcise the weirdness somehow, but it's a huge, beautiful estate in my favorite part of California: Santa Barbara County; excellent potential vineyard property, too)
Also, a flat in London (preferably on Kensington Palace Gardens or somewhere in Belgravia); a palazzo in Umbria, Italy; a place on the Upper East Side in NYC, and a ranch near Beaver Creek, CO. I'd probably want somewhere to stay in Philly, too, so a house in Chestnut Hill or Villanova, and a penthouse in Rittenhouse Square.
But, of course, I would be fine in a little house or apartment if it's warm and I have someone there to make it my home. The billion dollars would still be sweet, though. I'd be an awesome eccentric billionaire.