Which scientific discovery was most ahead of its time?

Cases where the scientific knowledge was actually lost and then rediscovered much later provide especially strong evidence w.r.t. the discovery counterfactuals. E.g. Hero's eolipile or al-Kindi's development of relative frequency analysis for decoding messages. Probably there are far more cases of this than we realize, because the evidence that someone somewhere once had the knowledge and then lost it has itself been lost; e.g. we could easily have just never rediscovered the Antikythera mechanism.

In scientometrics, there is a literature on "Sleeping Beauties"—papers whose relevance has not been recognized for decades, but then suddenly become highly influential and cited. The Einstein paper "Can quantum-mechanical description of physical reality be considered complete?" is a popular example of this.

ahead of their time, in the sense that if they hadn't been made by their particular discoverer, they wouldn't have been found for a long time afterwards?

This definition is surprisingly weak, and in fact includes some scientific results that were way past their time. One striking example is Morley's trisector theorem, which is an elegant fact in Euclidean 2d geometry which had been overlooked for 2000 years. If not for Morley, this fact might have remained unknown for millennia longer.

More engineering than science, but Turing's 1944 'Delilah' system for portable speech encipherment had no equivalent for more than a decade. (His biographer claims "30 years" but I don't know what he's comparing it to.) It was never deployed and was classified by the British government, so it had no impact.

AI Impacts' project on discontinuities in technological progress might have some relevant examples for this: https://aiimpacts.org/cases-of-discontinuous-technological-progress/

I don't recall the source, but I remember hearing from a physicist that if Einstein hadn't discovered the theory of special relativity it would surely have been discovered by other scientists at the time, but if he hadn't discovered the theory of general relativity it wouldn't have been discovered until the 1970s. More specifically, general relativity has an approximation known as linearized gravity which suffices to explain most of the experimental anomalies of Newtonian gravity but doesn't contain the concept that spacetime is curved, and that could have been discovered instead.

I think part of the "continuity" comes from the fact that things that were "ahead of their time" tended not to be useful yet and get lost. Or worse, perhaps several people had to independently come up with, and support, and learn about an idea enough to use it for it to be actually adopted, or it just ends up sitting in some tinkerer's basement or a dusty old tome.

So, you can flip this question: Which discoveries and inventions seem to have occurred after their time (e.g. they were technologically possible, the prerequisite ideas were pretty well known, and they would have been immensely useful practically in that time and place) and why didn't civilization get at them before?

Charles Babbage designed The Analytical Engine, that was a mechanical general purpose (Turing complete) computer, in 1837. This is remarkable, because it came a century before all the theory that was put in place by Turing, which inspired, and is at the heart of, today's computers. You can find a description of The Analytical Engine in Babbage's biography: "Passages from the Life of a Philosopher". His apprentice Ada Lovelace wrote some programs for it, becoming the first programmer in history.

This fact inspired a lot of Steampunk fiction, reasoning along the lines of: "What if the Analytical Engine was actually built and improved upon at that time? What if other non general purpose mechanical calculators like The Difference Engine followed the same development of the ones based on circuits we saw during the twentieth century?"

This post might be relevant to your question: https://kk.org/thetechnium/progression-of/

On Einstein:

What about great geniuses like Einstein? Doesn’t he disprove the notion of inevitability? The conventional wisdom is that Einstein’s wildly creative ideas about the nature of the universe, first announced to world in 1905, were so out of the ordinary, so far ahead of his time, and so unique that if he had not been born we might not have a his theories of relativity even today, a century later. Einstein was a unique genius no doubt. But as always, others were working on the same problems. Hendrik Lorentz, a theoretical physicists who studied light wave, introduced a mathematical structure of space-time in July 1905, the same year as Einstein. In 1904 the French mathematician Henri Poincare pointed out that observers in different frames will have clocks which will “… mark what on may call the local time. … as demanded by the relativity principle the observer cannot know whether he is at rest or in absolute motion.” And the 1911 winner of the Nobel prize in physics Wilhelm Wien proposed to the Swedish committee that Lorentz and Einstein be jointly awarded a Nobel prize in 1912 for their work on special relativity. He told the committee “…While Lorentz must be considered as the first to have found the mathematical content of the relativity principle, Einstein succeeded in reducing it to a simple principle. One should therefore assess the merits of both investigators as being comparable…” (Neither won that year.) However, according to Walter Isaacson, who wrote a wonderful biography of Einstein’s ideas in “Einstein: His Life and Universe”, “Lorentz and Poincare never were able to make Einstein’s leap even after they read his paper. Lorentz still clung to the existence of the ether and its ‘at rest’ frame of reference. Until his death in 1912, Poincare never fully gave up the concept of the ether or the notion of absolute rest. In other words, Einstein made a conceptual leap that Poincare and Lorenz could not make even after Einstein explained it.” But Isaacson, a celebrator of Einstein’s special genius for the improbable insights of relativity admits that “someone else would have come up with it, but not for at least ten years or more.” So the greatest icon genius of the human race is able to leap ahead of the inevitable maybe 10 years. For the rest of humanity, the inevitable happens on schedule.