When you switch on your computer, which of the following component affect first MotherBoard or SMPS???

SMPS because it only supply power to motherboard. So SMPS is first affected.

As smps is switching mode power supply,So it regulates power very often

SMPS is the first hardware in any PC cabinet in terms of Power supply connection.
Power supply to all the hardwares is provided by SMPS only as per capacity of the hardware.
So in terms of Power supply, SMPS will be first affected then the Motherboard
Pleas note: Its will happen only if SMPS works fine else No affect on Motherboard or other devices connected to SMPS.