Directly coupling high-speed generators to turbines or expanders enables compact conversion of low-grade thermal energy from industrial waste heat, geothermal resources, and small-scale renewable sources. For this application, this paper presents a multiphysics comparative assessment of six candidate topologies: three-phase and six-phase surface-mounted permanent magnet, NdFeB and ferrite interior permanent magnet, synchronous reluctance, and induction machines, all evaluated within a common 30 krpm, 750 V DC-link, water-cooled IEC 132 frame envelope. Two-dimensional transient finite element simulations are used to obtain electromagnetic performance maps, torque-speed characteristics, power factor, and efficiency, while thermal and mechanical analyses are used to assess continuous operating capability and rotor integrity. The comparison shows that peak electromagnetic performance alone is insufficient for topology selection, particularly at high speeds. For that reason, the results serve as a design-selection basis for high-speed generator topologies before detailed optimisation and prototype development.