I'm sorry, bur four letter words should not be used on this forum.
You said BEST (four letter word). That will surely get someones hackles up.
Most forges are lined with K-wool. 2" is BETTER than 1".
Pourable/rammable refractory is BETTER as far as durability.
Fire bricks come in different types ,and some are BETTER than others.
Satanite is GOOD, but bubble alumina is BETTER.
It really depends on the forge and its use as to the BEST trade off between cost, durability, and insulation .
For a general use knife forge ,I recommend 2" of K-wool and a coating of ITC-100.The floor should ,at a minimum, be covered in bubble alumina if there will be welding flux used in the forge. A firebrick/kiln lining floor is even BETTER if there will be lots of damascus welding.
Contact Darren Ellis for any questions (read his FAQ's) at
http://forgegallery.elliscustomknifeworks.com/
Stacy