1. Nazi's - it doesn't get any worse than that. Planned genocide on a grand scale. I cannot think of one redeeming feature - maybe autobahns.
2. Soviets - murdered more than the Nazi's and unlike many didn't bother to restrict themselves to other countries citizens, quite happy to kill their own. Slightly better as they at least educated their people and sponsored developments.
At their worst, their human rights record and level of oppression was worse than the Nazi's.
3. Belgian Empire - Just a shit show, no other words for it.
4. Japanese Empire - atrocities, chemical warfare, bio warfare. POW's, Kamikazes. They had it all.
5. Toss up, can't decide between Italian, Dutch, French, American, German, British, Ottoman, Spanish and Portuguese. Maybe Portuguese, they did little for their colonies and it's got to be pretty bad when the Spanish say they were terrible to natives by Spanish Empire standards of the time.
Italians- a lovely set of atrocities and chemical weapons use in Ethiopia. Did SFA for the natives
Dutch- fought a war complete with atrocities post WW2 when others were attempting to decolonize
Americans-Talk about eager, instigated two wars to get an Empire, less oppressive than many I suppose
Germans- some "lovely" work in SWA
British- Concentration camps in South Africa, planned starvation in India, probably less exploitative than some
Ottomans- see Armenian history
Spain- see gold, take gold, slaughter those opposing taking gold
Portugal- see above
France- fought bitter post WW2 colonial wars pathetically trying to hold on
Russian-lets start with state sanction pogroms and move on from there, milder than many though. They only exiled their opponents, not generally torturing and shooting them en mass like Mr Lenin and Stalin etc