In a similar spirit, looking back on the past 74 years, I suppose the computer might be a choice; however, it is not really one development, but a congeries of interpenetrating inventions: not only fast arithmetic, but cheap long-distance communication, video displays, satellites, and so on. If I had to choose one well-defined improvement, I think it would be plastic bags, which (IIRC) became cheap & widely available in the 1960s. Before then, if you went shopping, you usually carried off the merchandise in a paper bag without handles, which you had to put your hands under, so that your arms were aching when you got home. If you were packing a suitcase (respectable people didn't use backpacks), there was no convenient way of segregating things of different sizes or uses; now, you can put things in bags that reveal their contents, and pack those. If you wanted to put leftovers in the fridge, there was nothing for it but to put them in something that later had to be washed. If you were going hiking or (worse) boating, there was no good way to pack things so that they wouldn't get wet.
Most wondrous of all, plastic bags may well deserve the credit for the spectacular decline in insect pests during my lifetime. I spent most of my childhood in prosperous suburbs with bourgeois sanitation, but everybody made continual war using flyswatters, flypaper, and Flit guns. In those days, garbage was put out for collection without wrapping, in battered galvanized-iron cans with loose-fitting lids & (soon enough) rusty holes. The results were not as bad as in Mencken's reminiscences of the 1880s, when Baltimore had horses, backyard privies, & open sewers, but the change since then has been almost as spectacular. I still maintain a flyswatter, but occasions for using it are down to one or two a year. So late as 1960, when I lived in Cambridge, MA, I used to have to kill half a dozen mosquitoes before I went to sleep. (I could slap them in the dark with fair efficiency when they got near my ears.) These days, in Malden, a mosquito bite is a rarity. The improvement in public health due to cheap sealing must be considerable.
Sure enough, TANSTAAFL. I hear that there is by now a considerable floating island of plastic bags somewhere in the Pacific, and it's not good for the fishes.