I think thorium MSRs are the most promising of the GenIV designs. However, the biggest engineering challenge with them is with the U-232 being produced. U-232 decays into Thalium-208 and gives off very high energy radiation which would kill anyone near it, as well as damage any electronics in the vicinity (this is also why making bombs out of thorium are not as big a threat). But if this can be overcome I think it'd be a great source of power.