I use Trukke boots. The current model is called Thundersnow III. They are marketed as a snowmobile boots but they work well for me with platform pedals.
I got these after a long search for a double insulated boot suitable for winter biking. The Trukke boots have a stiff sole, unlike normal shoepacs from Sorel or Baffin. The Trukke boots stand up to pedaling unlike the others which are too soft.
They may seem like overkill, but they work great when there's snow on the ground. In my experience the lightweight insulated biking shoes marketed by Sidi, Lake, etc. aren't near warm enough unless you live in Florida.