The best conductor of thermal energy among the given options is vanadium (V).
Vanadium is a transition metal and generally, transition metals are excellent conductors of heat and electricity. In contrast, polonium (Po) is a metalloid, germanium (Ge) is a semiconductor, and argon (Ar) is a noble gas and a poor conductor of heat because it is not a solid. Thus, vanadium stands out as the best conductor of thermal energy among the listed elements.