This answer is very straight forward, it's the car or motorcycle of course. Never mind half atoms and the transistor. Yes I know, these date from earlier centuries depending on which expert or patent office you talk to, but don't they count as evergreen content, ha.
Seriously though, I think it's simply a choice of 2, the internet and the mobile phone. I can't decide which though I think the internet has the edge.
Which other inventions have served to bring mankind to a closer universal understanding of each other. Yes, absolutely, mankind still has a huge way to go but surely with these 2 tools alone, world peace and cultural understanding has to be a realistic goal.
Just a mad little thought, from a mad little Irishman,