The principles in network monitoring

I'm kind of new here, so first and foremost: Hi everyone!

The size of the network in our company has recently exceeded the limit to manage it in a simple way, so we face with the choice of an appropriate monitoring system. It's all about effective monitoring of devices (switches, printers, etc.), servers (Windows and Linux), some services and applications, etc. - a total of about 250 devices (this number probably will increase).

Wanting to choose wisely, I would like to ask you, what is the most important from your POV in network monitoring and monitoring of what parameters in your networks brings you the most benefits? (please attach the size of your net).