The set 4N35, 4N36, 4N37 are popular and versatile optocouplers for isolation of circuit blocks.
The most important feature is the Isolation test voltage 5000 VRMS that means 5KV. But PCB Design is very important to get this isolation for your products too. The PCB area under the Optocoupler can be a Slot that is routed in a CNC Driller or CNC router.
That way Air Insulation makes Isolation good. The Line of Isolation has to be observed in all SMPS under Magnetics and Optos. Creepage of HV can occur due to Grime and Humidity on PCB, use RTV Silicone Rubber or Epoxy impregnation to make your product Industrial or Military Grade. - delabs design Notes
Other Features - Interfaces with common logic families, Input-output coupling capacitance < 0.5 pF, the low level capacitance means you can switch this opto at high speeds.