The most expensive real estate neighborhoods in the world can vary from year to year based on market trends, but here are some examples of some of the most expensive neighborhoods as of 2021:

1. Hong Kong - The Peak: Hong Kong's The Peak neighborhood is known for its stunning views of the city and the surrounding area. The average price per square foot for a home in The Peak is around $3,100.
2. London - Knightsbridge: Knightsbridge is home to some of London's most exclusive shops and luxury brands. The average price per square foot for a home in Knightsbridge is around $4,600.
3. New York City - Central Park South: This neighborhood is located right next to Central Park and is home to some of the city's most luxurious apartments and hotels. The average price per square foot for a home in Central Park South is around $5,200.
4. Paris - 7th arrondissement: This neighborhood is home to some of Paris's most famous landmarks, including the Eiffel Tower and the Champ de Mars. The average price per square foot for a home in the 7th arrondissement is around $1,500.
5. Tokyo - Minato: Minato is a central Tokyo district that is home to many of the city's top businesses and embassies. The average price per square foot for a home in Minato is around $2,000.

It's worth noting that these are just a few examples of some of the most expensive neighborhoods in the world, and the actual cost of real estate can vary widely within each neighborhood.