If this is on personal opinion, I would say the initial creation of the telephone. Even though a smartphone practically kickstarted a whole era to technology that is still going on till this day, it was the telephone that started this all. If it wasn’t for the idea to create a phone that is more advanced than a box attached to the wall, none of these advancements would’ve happened at all. (Although that is uncertain, things do tend to have a way of working out in the end. If someone didn’t invent it, another person would’ve thought of it sooner or later.) But that is only if we’re talking long term, which I prefer because there was no limit to when the results of impact started in the prompt stated above, yet if we were to speak short term and the faster one, the credit would go to the smart cellphone. But we aren’t, because the question never specified ;)
