The best conductor of thermal energy among the options given is polonium (Po).
Polonium is a metalloid and, like other metals, generally has good thermal conductivity. In contrast, argon is a noble gas and does not conduct heat well, while germanium, although a semiconductor, is not as good a thermal conductor as metals. Vanadium, being a metal, does conduct heat, but polonium, as a metalloid, generally has better thermal conductivity compared to most elements in that category.
So, the correct answer is polonium (Po).