It's not the thickest on paper I don't think but the relatively newish NorthShore Supremes have a sort of stiffness to the padding that (to me at least) makes them feel somehow thicker than they really are.
That, and their absorbency is INCREDIBLE.
As for straight up thickness though, I would think you would want to look for a diaper that is more pulp heavy in terms of pulp/SAP ratio, as pulp tend to contribute to the majority of a diapers thickness, whereas SAP is highly absorbent, but comparatively smaller in size. The Abenas come to mind as a diaper that is primarily pulp based (and thus, thicker).