'Forge world is a term for the numerous worlds that are directly controlled by the Adeptus Mechanicus. All have in common that they are completely dedicated to the manufacture of the various machines of the Imperium, the pursuit for and preservation of (ancient) knowledge and the worship of the Machine God.'
The argument could be made that since they've been taken over, they're no longer Forge worlds, but if we're including planets eaten by 'nids and traitor Forge worlds, Neffethyl and Urdesh are good candidates, for the simple reason that:
:teehee:
I've not read the books in which the Blood Pact appear (Abnett?), so I can't really offer anything more than the White Dwarf quote to support my suggestion.