Estimated value: $2.9 billion

With 775 rooms, including 52 royal and guest bedrooms, 78 bathrooms and 92 offices, the Queen's London home still holds the top ranking for the most expensive house in the world. Although the British royal family owns many castles and estates throughout the United Kingdom, Buckingham Palace has been the monarchy's official residence since 1837. Queen Elizabeth II spends her annual summer holiday at Balmoral Castle in Scotland's Highlands.

Estimated value: $1-2 billion

Designed in consultation with Chicago-based architecture firm Perkins & Will and Santa Monica-based hospitality design firm Hirsch Bedner Associates for Indian billionaire Mukesh Ambani, this 400,000 square-foot residence in Mumbai's Cumballa Hill neighborhood is 27 stories high. The tower, which can sustain an earthquake of magnitude 8 on the Richter scale, features six floors devoted to car storage, a car service station, a temple, a 50-seat movie theater and nine high-speed elevators.

Estimated value: $750 million

Owned by Lily Safra, widow of Lebanese born Brazilian banker Edmond Safra, this 11-bedroom, 14-bathroom estate is perched on 50 acres in France's Alpes-Maritime department of its Cote d'Azur region. It features a commercial greenhouse, helipad, outdoor kitchen and stunning pool and was the setting for Alfred Hitchcock's 1955 film "To Catch a Thief." Villa La Leopolda is named for its original owner: King Leopold II of Belgium gifted the estate to his mistress Blanche Zelia Josephine Delacroix in the early 20th century. The home was redesigned by American architect Ogden Codman Jr. in the 1920s.

Estimated Value: $410 million

Built in 1830 then purchased in 1904 by Belgium's King Leopold II, this mansion was considered to be the most expensive house in the world on the market, when it went up for sale in 2017 for $410 million by Davide Campari-Milano S.p.A of the Campari Group, the property's current owner. Set on 35 acres of manicured gardens and named for the cedar trees on the grounds, the 18,000 square-foot, 14-bedroom estate boasts an Olympic-sized swimming pool, a large stable and decadent interiors (think ginormous crystal chandeliers, intricate gilded woodwork and 19th century oil paintings) fit for the opulence of the home's Belle Epoque heyday.

Estimated value: $390 million

Designed by Hungarian architect Antti Lovag and built between 1975-1989, the "Bubble Palace" features a series of round rooms that spill down a rocky cliff overlooking the Mediterranean Sea. while Lovag was reportedly inspired by man's earliest dwellings in caves, the seaside estate not short on modern "comforts," featuring three swimming pools, several gardens and a 500-seat amphitheater on the hillside grounds. Bubble Palace is currently owned by Italian-born French fashion designer Pierre Cardin as a holiday home and has been the setting for many a swanky event: In 2015, Dior showed its cruise collection at an indoor/outdoor fashion show at the sprawling estate.

Estimated Value: $330 million

The 35,000 square-foot apartment at the top of the Tour Odeon, developed by the Groupe Marzocco and designed by architect Alexandre Giraldi, spans several floors and comes with its own private elevator. The private luxury doesn't stop there: A water slide coming from an upper floor into an infinity pool with 360-degree views of the surrounding landscape makes this penthouse pretty spectacular.

Estimated Value: $250 million

T. Boone Pickens' 65,ooo acre ranch boasts a lodge, a private airport, a pub, an 11,000 square-foot bird dog kennel with room to house 40 dogs, a chapel, golf fairways and greens and nearly 20 man-made lakes. The oil tycoon moved his childhood home from Oklahoma to the ranch in 2008.

Estimated Value: $248 million

Ira Renner's giant limestone Italian Renaissance-style home sits on 63 acres of beachfront property in the Hamptons. The 62,000 square foot mansion has 29 bedrooms, 39 bathrooms and an expansive 91-foot dining room. The property is also home to all kinds of luxury leisure spaces, such as a basketball court, a bowling alley, squash courts, tennis courts, three swimming pools and a garage that can hold up to 100 cars.

Estimated Value: $238 million

Hedge fund billionaire Ken Griffin made history when he purchased the 24,000 square-foot penthouse in this Robert A. M. Stern-designed skyscraper overlooking New York's Central Park for $238 million, making it the most expensive home sold in the U.S. The 79-story tower is distinctive due to its color: the building is sheathed Alabama Silver Shadow limestone. It's also known for its luxurious amenities, which include a spa, an athletic club, private dining and entertaining facilities and a private motor court.

Estimated Value: $222 million

Owned by Indian steel tycoon Lakshmi Mittal, the 55,000 square-foot 19th century mansion was originally built as two semi-detached homes. After housing the Egyptian and Russian embassies for many years, the buildings were purchased by developer David Khalili, who merged the two into one downright palatial residence.

Estimated Value: $160 million

Once the home of the late television producer Aaron Spelling, this 56,000 square-foot home boasts 14 bedrooms, 27 bathrooms and was once the largest home in Los Angeles County. The over-the-top style starts right at the front door with a 40 foot-high foyer designed after the O’Hara Plantation, Tara, from "Gone with the Wind." The estate counts a gym, a wine cellar and tasting room, a beauty salon with three stations and massage and tanning rooms among its amenities.

Estimated Value: $150 million

Developer Bruce Makowsky has thought of everything a billionaire might want at home at this 38,000 square-foot Bel Air mansion. In addition to the more traditional trappings of two master bedrooms, 10 guest suites, 21 bathrooms, three kitchens, five bars and even a massage room/wellness spa, the estate boasts more outrageous amenities like: a helipad, an infinity pool, a candy wall, two wine cellars, a four-lane bowling alley and a pop-up outdoor theater. Had it sold for its original listing price of $250 million, it would have been the most expensive home to sell in the United States.

Estimated value: $129 million

Owned by billionaire developer Jeffrey Greene, this Mediterranean-style estate has a whole lot to love about it, starting with spectacular views of Los Angeles and the surrounding canyons. Other sublime features: a vineyard set on the hillside, manicured gardens and a waterfall, a Turkish spa, a DJ booth with revolving dance floor and laser-light system, a 50-seat movie theater and parking for 150 cars.

Estimated value: $125.5 million

It took Bill Gates seven years — and $63 million — to build his 66,000 square-foot waterfront estate in the Seattle suburb of Medina. Not surprisingly, the home is packed with high-tech amenities, including a sensor system that helps regulate temperature and lighting, an underwater music system in the pool and computer screen-displayed artwork that can be changed with the click of a button. The mansion reportedly also boasts six kitchens and 24 bathrooms.

Estimated Value: $88 million

Known for her privacy, Oprah has not shared many details about her sanctuary on the California Coast — but she has shared the extensive rose garden she designed with master rosarian Dan Bifano with Veranda. Oprah also worked with legendary designer Rose Tarlow to redecorate the 23,000 square-foot Georgian mansion.