The ancients were responsible for several scientific achievements that were eventually lost to history. Of these technologies, which would have made the biggest historical impact if, rather than being lost, they were retained, refined, and used extensively by successive generations and proliferated to other cultures?
I got most of this list from
the Wikipedia article on the subject and related links, but to summarize...
Antikythera mechanism: Long a subject of conspiracy theories, this Greek clockwork contraption dating back to several centuries before the birth of Christ seems to have been an early analogue computer used to study astronomical features. We only have the one artifact, and while there hypothetically could have been more, we do not have evidence of anything like it for ages afterwards.
https://en.wikipedia.org/wiki/Antikythera_mechanism
Damascus steel: Produced from the late Classical Era to almost modern times, these ingot-based blades were uniquely strong.
https://en.wikipedia.org/wiki/Damascus_steel
Flexible glass: This one may be apocryphal, but the idea does not seem more outlandish than some other entries on this list. It is said that a Roman inventor produced glass resistant to shattering - that is, if dropped, it would merely bruise in such a way that could be easily repaired - in the first century AD. Fearing economic crisis if widely adopted, Emperor Tiberius had the inventor killed.
https://en.wikipedia.org/wiki/Flexible_glass
Greek fire: Famously used to defend Constantinople from Arab invaders, this Byzantine weapon was a chemical mixture that set water ablaze, capable of being propelled from a Medieval flamethrower.
https://en.wikipedia.org/wiki/Greek_fire
Iron pillar of Delhi: Built sometime during the lifespan of the Gupta Empire in India, this giant rod is abnormally resistant to rusting.
https://en.wikipedia.org/wiki/Iron_pillar_of_Delhi
Roman concrete: Developed in the Roman Republic and used well into the Imperial era, this material was incredibly durable.
https://en.wikipedia.org/wiki/Roman_concrete
Other: I left off a few from the aforementioned list, especially the more recent inventions, but if you know of any lost technology, whether it was lost permanently or merely temporarily, which you feel could have made an even bigger impact, please share!