A teleporter! Just imagine how useful it would be if we all could just teleport ourselves to any place in no time. We could go to the beach in Italy for an afternoon when the UK is grey, cold and rainy again. Or you could visit friends abroad really easily. No more long commutes. Presumably we wouldn’t get stuck in traffic ever again. Oh, that would be so nice!
This is actually a very hard question. I would probably have a different answer tomorrow, or the day after. For today, I would say inventing a better form of government. “Better” might mean more closely representing the demographics and concerns of the population, more efficient use of time and resources in the business of governing, greater use of empirical evidence in policymaking, an emphasis on longer-term planning, and being less sexist, racist, abelist, and generally less committed to sustaining structural inequalities.