Which network device has the primary function?

I'm curious, which network device stands out as having the most essential function in a system? Is it the router, which directs data packets to their destinations, or the switch, which facilitates communication between devices on the same network? Could it be the firewall, safeguarding the network from potential threats? Or perhaps it's the modem, bridging the gap between the network and the outside world? I'd love to know your thoughts on which device truly holds the primary function in a network setup.