In reply to 19G:
Polartec Thermal Pro is an inside brush that exists in several Polartec fabric finishes and it does offer one of the loftiest fleece around. An even loftier Polartec fabric is the High Loft - a hair, fur like finish.
If you really want a good mid layer, especially for active use, go for the Power Dry (if you run hot), Powerstretch (warmer) or Powerstretch Pro (warmest). These fabrics have some stretch to them so are more fitted and will wick sweat faster.
Do note that those are midlayers only, and without a shell won't be useful as you will lose the heat retention with the lightest of wind.