What’s the good frequency of switching mode power supply to have good efficiency?
It depends on the power level and how good is good. The power grid runs on 50/60 Hz although not switched for a reason.
GW HVDC substations switches at about 50-2000 Hz. MW traction converters for trains around 1 kHz. Multi kW-power supplies for inductrial applications around 5-10 kHz. kW level high end amplifiers and welding equipment at about 50-100 kHz. 100 W laptop chargers and the like at about 60-200 kHz. Small 10 W cell phone chargers at ~500 kHz. Integrated single-W converters inside computers and simular in the MHz region.