If you ask me? Mobile/WiFi internet… The way and amount of time we use our phones had changed A LOT since their diffusion. I guess the release of the iPhone changed our idea of what a phone is too
Edit: when I say modern world I’m referring to the last 50 years. So stuff like “the electricity” or “the telephone” doesn’t count.
It depends on what you mean by piece of technology I guess, since what we have now is a culmination of thousands of awesome tech over the last few hundred years.
If I were to choose one thing, I’d say the telephone. It’s the predecessor to the internet, and suddenly communication between people was instant rather than messages that’d take forever (or morse for the places that had it).
It probably changed the world forever, being able to talk to someone in a completely different country and share something quickly.