Opinion of America's greatest geopolitical foe

[Frodo]

Russia, as it is they who seek to undermine us as a world superpower and constitutional republic, and who never reconciled themselves to having lost the Cold War.  Only it poses an existential threat. 

China, while also a threat, is only a potential regional foe who seek only to define their own sphere of interest.  They do not (yet) have aspirations to world domination that I am aware of.   

Ditto for Iran.