  The anomalous Hall effect due to the surface conduction band of 3D
topological insulators with an out-of-plane magnetization is \textit{always}
dominated by an intrinsic topological term of the order of the conductivity
quantum. We determine the contributions due to the band structure, skew
scattering, side jump and magnetic impurities on the same footing,
demonstrating that the topological term, renormalized due to disorder,
overwhelms all other terms, providing an unmistakable signature of $Z_2$
topological order. Uncharacteristically, skew scattering contributes in the
Born approximation as well as in the third order in the scattering potential,
while in addition to the side-jump scattering term we identify a novel
intrinsic side-jump term of a similar magnitude. These, however, never
overwhelm the topological contribution.
