What do you mean "pre-WWI"? Do you mean the entirety of human history or just a few years or what?

This isn't exactly a question that any historian or academic really can 'answer', given that there isn't a quantifiable answer. Even you are asking for an opinion in your post. So I don't know that this is appropriate, but in my opinion, I would point you not to a weapon or a tactic but a simple tool: the stirrup. The affect of man 'harnessing' the power of the horse forever changed warfare, and had a universal impact on the scale of the proliferation of gunpowder weapons. The difference being the horse has defined several thousand years of human history insofar as war is concerned, as opposed to the, say, 5 maybe 6 centuries that warfare has revolved around firearms. While people had domesticated the horse several thousand years ago, as far back as the 4 thousands BCE, the earliest actual evidence of a stirrup or likewise is from 700's BCE Assyria. Since roughly that time, nearly any decisive military effort involving horses, from the Greeks to the Romans to the Arabs to the Mongols, would involve in some way shape or form the use of this little tool. The horse is simply tied to the history of humanity, particularly in the realm of warfare, like no other creature or tactic. Prior to machination and industrialization, there simply was no alternative to the sheer mobility, agility, and impact of horseback warriors on the battlefield. And this was the case for thousands upon thousands of years, made possible by this little foot holster that allowed man to master this legendary creature. So, that's my candidate.

Idk if this would be the greatest, but the invention of the bayonet pulled the modern world out of the Age of Pike and Shot, so that was a "recent" big one.

Then again I'd argue it was the Spanish and their Tercio Square that pulled Europe out of the Late Middle Ages. So that was important too...(I don't mean pulled them forward culturally or technologically, I'm strictly speaking military tactics.)

I'm not really sure if there is one "greatest" advancement.

Sorry, I've removed your question. From our subreddit rules:

"Poll"-type questions aren't appropriate here: "Who was the most influential person in history?" or "Who was the worst general in your period?" or "Who are your Top 10 favourite people in history?" If your question includes the words "most" or "least", or "best" or "worst" (or can be reworded to include these words), it's probably a "poll"-type question. These questions do not lend themselves to answers with a firm foundation in sources and research, and the resulting threads usually turn into monsters with enormous speculation and little focussed discussion - and, as such, are banned here.

In other words, as other posters have pointed out, "greatest" is not really something historians can assess and therefore this question doesn't have a scholarly historical answer.