I reckon the one with the most clout is Lösch.
This is why:
Lett is clearly two sanwhiches short of a picnic.
Herd is too traditional, stuck on the past, old, and a bit slow.
Sanderson is too new.
Jackson is a bit of a joker and doesn't have really important oversight roles, translation is his main focus.
Splane is possibly a big influence, as he's the studious one, and is probably responsible for finding new light. He is on the writing committee that produces magazine content.
Morris is on a number of important committees, not least the teaching committee, which supervises preparation of teaching materials, and which he clearly treats as his personal fiefdom, seeing that his child-indoctrination videos get made and that conventions include more and more video. But... he's just a bit of a loose cannon. I can't imagine him having much personal influence on people like Lösch and Splane.
This leaves Lösch, cool, sober and business-like, and on committees that really influence the organisational stance and structure. The Personnel Committee controls Bethel, the hub of the organisation (the power to fire people) and the Service Committee. The Service Departments in Bethels around the world are the power bases of each Branch. Service brothers are like the "elders" for the whole country. I'm sure this is where the power lies. All inter-branch communication flows through it.
On the other hand... Stephen Lett is on three committes, two of which hold the purse strings of property and convention revenue, and the other is the loftily named "Coordinators Committee" so... But come on... Stephen Lett?