Although clathrate materials are known for their small thermal conductivity, they have not shown a large thermoelectric power factor so far. We present the band structures of type VIII Si, Ge, and Sn clathrates as well as the alkali and alkaline-earth intercalated type VIII Si clathrates. Our calculations revealed that this group of materials has potentially large power factors due to the existence of a large number of carrier pockets near their band edges. In particular, we calculated the charge carrier transport properties of Si₄₆-VIII both for n-type and p-type materials. The exceptionally high multi-valley band structure of Si₄₆-VIII near the Fermi energy due to the high crystallographic symmetry resulted in a giant power factor in this material. It was shown that the intercalation of Si₄₆-VIII with alkali and alkaline-earth guest atoms shifts the Fermi energy close to the conduction band edge and, except for Be₈Si₄₆ and Mg₈Si₄₆, they weakly influence the band structure of Si₄₆. Among these clathrate systems, Ca₈Si₄₆, Sr₈Si₄₆, and Ba₈Si₄₆ showed negative formation energy, which should facilitate their synthesis. Our results imply that the intercalation affects the conduction band of Si₄₆-VIII more than its valence band. Also, interestingly, the type VIII clathrates of Si₄₆ and its derivatives (except Be₈Si₄₆ and Mg₈Si₄₆), Sn₄₆, and Ge₄₆ all have 26 carrier pockets near their valence band edge. Among the different derivatives of Si₄₆-VIII, Rb₈Si₄₆ and Ba₈Si₄₆ have the highest number of electron pockets near their band edges. The thermoelectric power factor was predicted using a multiband Boltzmann transport equation linked with parameters extracted from density functional calculations. It was shown that both the increment of charge mobility and the existence of multiple band extrema contribute to the enhancement of the thermoelectric power factor considerably. Such a large power factor along with their inherently low thermal conductivity can make this group of clathrates promising thermoelectric materials.