The POP
You should attempt to locate your main closet close to the Point of Presence (POP) to the Internet.
The POP is the location where all of the telecommunications for a given building are located.
It is where a LAN is connected to the Internet via the telecommunications company's dial-up or leased lines.
Placing the MDF near the POP facilitates your network's connection to the Internet.
If you plan to have multiple wiring closets, in support of a larger network, then your MDF should be located in the POP room if possible.