Azure Virtual Networks (VNets) provide the fundamental building block for the private network in Azure, enabling secure communication between Azure resources. They also allow for network isolation which is important in improving the security and organizational structure of an Azure environment. Therefore, leveraging Azure Virtual Networks is best for achieving network isolation and secure communication. An ExpressRoute is used to create private connections between Azure datacenters and infrastructure on your premises or in a colocation environment, which is not primarily for network isolation between Azure resources. Azure Virtual Desktop is a desktop and app virtualization service and does not relate to network isolation. VM Scale Sets are for scaling applications and have no direct relation to network isolation or secure communication between parts of an application.