> Of the 3, if we ask "which of these is closest to being an imperial nationalist power?" the answer is the US. It isn't remotely close.
Of the 3 countries, one of them is currently engaged in a war which is explicitly about achieving its imperial ambitions, which is actually the fourth or fifth such war (depends on what you consider separate wars) it has conducted in my lifetime. Instead, you have picked one of the two countries that has yet to conduct one such war in my lifetime (the invasion of Panama was before I was born).
> They are struggling mightily to promote their interests abroad because of all the US bases people keep tripping over, and US-based systems of global governance. Their armies are struggling to move more than a few hundred kilometres from the borders that the US recognises. The US military goes where it wants and does what it wants.
If you're focusing on the results of attempted wars, even classifying all US military actions in recent decades as imperialistic wars (which is a mistake), you'll find that the US has been less successful than even Russia (which is actually 3-for-5 in its recent wars). The idea that the US (/US-allied) bases are deterring Chinese and Russian influence is also pretty laughable--see, for example, Mali, Djibouti, Solomon Islands for things I can think of the top of the head.
> A dictatorship just couldn't sustain a global empire the way a democracy like the US does (or the British before them).
The Roman Empire lasted about 270 years as a dictatorship, which is longer than the combined British/US hegemony [1], even if you take the charitable view that the period of British global hegemony starts from the end of the Seven Years' War. Of course, individually, our two model democracies each don't manage to last 100 years before fissures in their hegemony appear.
> If Russia wanted to win more wars it'd need to use a political system more like the US's
Why should Russia learn from a country that has a worse win rate than it? The US hasn't been successful in its recent military escapades, whereas Russia has had moderate success.
[1] The US does not have an empire, it has a hegemony. Yes, there is a difference, and it matters.