I would say 'Condoms' without it there would have no space on the earth to do any thing else. Ofcourse it is electron....
I would say electricity, because it made many more things possible, but if you think about it, electricity was not really invented but rather it was discovered how to use it in a way that can be controlled. Electricity exists and thankfully it makes such a big difference.
E85, I know that it's not what most people would call important but one day it might be the life blood of the planet, although I think that the gov't is suppressing technology that would better use the power of oil.
Computer games are the most important and useful invention of the last 500 years. A way to pass time. Escapism. But proberly not though.