Thief Bishop. And if there's a biggest Thief Bishop (like the pope originally), it's probably Heidi. Though I don't think anyone will ever get the same level of dominance and centralization, given that a secretive mystery cult of a god of sneaking and rebellion is not very conductive to such. It's always going to be a collection of equal high priests, with some high priests equaller than others. But even building consistent power structures is difficult because the nature of most doings means that while Altdorf and other big cities will be more important, the people actually leading often don't stick around.
Heidi sticking in one place for a long time is kind of an interesting development in that way. Especially if Ranald the Dealer becomes an official thing. A merchant high priest will have have cause to stick around and be able to publicly exert power, so that could lead to more structure. But I don't think that could ever claim primacy. On public, 'official' policy, but the internal politics of the cult that produce the external policy will be heavily influence by the other facets.
Anyway, I do realise it's a joke, but there's some interesting thoughts related to it.
