If you go back far enough it might be the wheel. Think about it....without wheels we'd probably still be walking. No carts, wagons, stage coaches, cars, nothing. We'd spend our lives just dragging stuff around.
Maybe the bow and arrow, and later gunpowder? Then you could defeat those pesky 'ol Huns trying to storm your castle without having to ruin you nails in hand-to-hand combat.
The telegraph / telephone? Then you could communicate with friends or do business with people hundreds of miles away. The common man began to understand there was a big wide world out there.
The harnessing of electricity? Then we could see and do things after dark.
Oh....these were huge!....trains and automobiles. Where they were originally rich man's toys, Corny Vanderbilt and Henry Ford put us all on wheels. Now we could work and play and make stuff for people far away from where we lived. Until then most people grew up, married and raised families, worked and eventually died within maybe 25 miles of where they were born. Easy transportation opened up commerce, created jobs, and raised the living standards of the masses.
The airplane? A personal favorite of mine, it did the same as the automobile, but over longer distances.
Personal computers and the internet? Now we had the knowledge of the world at our fingertips.
And of course there are probably regional favorites, too. I'm sure If you live in the Mid-East you're probably still amazed by toilet paper. And judging by the way they like to throw them, rocks.
NOTE: This is NOT me, although the resemblance is striking.
But for me, a guy who grew up in the South and has become increasingly intolerant of heat, my personal favorite invention of all time is air conditioning.
Definitely, air conditioning. ;)
What say you?