Luanda, the capital of oil-rich Angola, tops the list of most expensive cities for expats for the third year in a row, according to a ranking released by consultancy Mercer on Tuesday. For expats, imported goods and safe living conditions come at a steep cost in Luanda.

The top five most expensive cities have remained relatively stable. Moving from No. 3 to No. 2 this year, Hong Kong, with its currency pegged to the U.S. dollar, is known for having one of the most expensive housing markets in the world. Zurich, Geneva and...