Based on this campus design, which layer is the most appropriate to be designed as...

Based on the campus design provided and considering an EVPN VXLAN Fabric, the most appropriate layer to be designed as a Border Persona would be the "Services Aggregation" layer. In an EVPN VXLAN architecture, the Border or Border Leaf nodes provide connectivity to external networks, such as WAN, internet, or private connections. They are responsible for routing traffic into and out of the VXLAN fabric and typically also handle services like firewalling, load balancing, and other network services.