In the US: ISP & Healthcare

Banking! If there are meaningful competitors then that implies there is meaningful competition. But many financial services firms have been making huge windfall profits by creating a wide interest rate spread even though they would still be (very) profitable with much lower spreads and their customers have a clear incentive to go with a competitor offering a better interest rate. So clearly there is not effective competition in this market.

Anything with a natural monopoly.

Utilities (water, electric, telephony/cable/data services) or where having two of something makes no sense given physical limitations (Transportation sector, etc.)

Social networks, search engines.