I can not really decide between the knife and the ability to make fire, but I guess I'll take the knife!
Deep inside nature programs us to remain our species, for that we are to survive ourselves, to eat, to sleep and to have children. Anything that helps us in that is truely important. So if you would like to see what realy is important, try to live without! I'm sure, your life will be boring without internet, work will be hard without the wheel, but have you ever tried to live a day or even a week without using a knife?
(says Marisa when it is 105 F / 41 C outside in the shade).
I'd say electricity, without it most of our current day things would not be here. No air conditioner, no computer....
And without a computer, there would be no need for an internet.
No fire no silicon to make any electronics
No fire, no tools from iron or any other metals, including knives
No fire, no pizza. I rest my case!
I would have to say it is the computer. Without them, we would have never gotten to the moon, figured out how to drill for our oil and gas, make knives and everything else better in our lives.