When it comes to contemporary imperialist assholery the US is #1 by far. Since after WW2 they've been nothing but shitting on humanity, between the CIA installing puppet dictators to wars (open and secret) killing tens of millions. The list of countries they have fucked with is too long to post here.
And now their former closest allies are annexation targets.
The USSR was a shitty little bitch during her heyday, too. Worse than the US? I'm not sure. Eastern Europe and South America probably have a major difference of opinion there.
In the last few decades I feel like Russia may have the US beat?
Yes. The vast extent of the negatives of maintaining a brutal Empire and the dedication towards thoroughly punishing anyone going against that system makes the US Empire the worst country on the planet.