It's difficult to judge which regimes are the most oppressive for there those whose doings are easily reported upon, and others where journalists are not welcome and reliable reporting of a situation is just not possible. Americans do not appear to feel they are in an oppressive regime, although there is risk of this where Supreme Court has the last say in vital matters to the populace but is not a voted-for body.
China is the largest population in the world and so there are more people for a leader to manage and to oppress if that is the regime. It does still allow its people to travel and so woud seem less oppressive than a regime where travel is more strictly regulated.
No. The U.S. is not considered an oppressive regime at all. It is known as the land of the free and the home of the brave. I live in the U.S. and I think this must be a typo. Who would think the U.S. is oppressive? Yes, we have problems with racial and gender inequality, etc.
but we are by no means oppressive. I really can’t speak for Russia. I think at one time they may have been considered oppressive but are they the most oppressive regime in the history of the world? I doubt it.