I'd say it's tied between Relics & Rituals (mostly for Ritual Magic) and Alchemy & Herbalists, with Nyambe starting to move into position due to recent events/location. Mercenaries, the PsiHB, BoVD are just EA: Battle Magic (War... Lots and lots of war...) a notch behind. I expect Fields of Blood and/or Dogs of War to hit that tier as well.