Hands-down it's Frostgrave. The Knights and Wizards boxes (and even to an extent the basic soldiers and the Ghost Archipelago box) are nothing short of superb with bits and arms and equipment, packs, pouches, accoutrement of every variety. Most importantly, their bits fit onto most other ranges - excluding perhaps Perry (which are smaller than other lines).
Frostgrave bits work on larger models like the Fireforge lines, and I believe are a match for Oathmark which I don't own so I can't comment on. The later Frostgrave kits (again, namely Knights and Wizards) are much crisper and finer detailed than some of the earlier Frostgrave boxes.
A couple pics of random minis I assembled just using the Knights box.