Are you familiar with Walter Scheidel at all? His new book, The Great Leveller: Violence and the History of Inequality from the Stone Age to the Twenty-First Century, offers a compelling (albeit grim) answer to your question.

Scheidel examines economic inequality through history and judges that it is at its lowest during enormous wars, epidemics and pandemics, anarchy and societal upheaval. That's because in most situations, the rich have the farthest to fall, the most to lose. The poor are poor regardless of conditions.

Now, if you want to examine factors other than the economic ones, I suggest Steven Pinker's Better Angels of Our Nature, which suggests the world has grown more peaceful and egalitarian as time has progressed. Pinker argues that globally, violence has declined if you examine the topic by proportion rather than absolute figures. The Earth has more people on it, so the tolls of wars might rise (WWI, WWII as examples), but as a proportion of global population, they make less of an impact. Think of King Phillip's War, which depopulated swaths of New England, or the Taiping Rebellion/Revolution, which killed millions in China during the middle of the 19th century. That's to say nothing of the Mongolian wars, which actually made the Earth's population decline overall.

If you combine these two arguments, you could make the case that the world was "most equal" in late 1945, shortly after World War II, when the ravages of that war caused an economic levelling effect both in the conquered and the conquerors. The institution of Western democratic practices by the advancing Allies also led to egalitarian laws and regulations, with the extraordinary caveat that this was done at gunpoint.