definitely electricty/the ability to generate/harness it.. from that one thing, spawned EVERY electric-powered appliance/gadget we have now. and thats what, about 70% of the stuff we have.
we wouldnt be exchanging opinions on serebii right now if it wasnt for electricity!!
but then i remembered that without the ability to effectively and efficiently handle fire, the ability to control electricity wouldnt exist as well. so it would be ultimately the ability to harness fire.
Any invention that expands our ability to learn more,or learn at a faster rate.That way we can make more useful gadgets and innovations.First there was writing,then books,then libaries,and so on until we got the internet.Here are some other useful tools
Plumbing-For a while it had some kinks in its design,but overall it was important.No longer did our biological waste(which,being a mammal,is a lot) have to be chucked in a pit,pile or just on the ground(which is what people did back in the Middle Ages).
Any fuel-It stopped us from having to use slaves and manual labor so much.Fossil fuels have been around for millions of years,but we were the first ones to use them.Okay,now they're running out and polluting,but for the time they were valuable.Electricity gave us a way of storing energy.And there's some new nifty power sources that could help out
Machinery and tools-If it weren't for tools,our species would've probably died out long ago,since we're a bit on the scrawny side compared to the other animals.Machines were useful since they did a whole lot of work for us
I would say the wheel, mainly because it's the basis for many other inventions. Computer? Need the gear, which is based off the wheel (and currently is very helpful in electricity in general). Plumbing? Thank screws for keeping it together, which is basically a variation of the wheel. I would say the only major thing that isn't based off the wheel/circular design is fire, and we didn't even truly invent it; we just learned how to harness it.