Mankind's greatest achievement? I'm gonna have to say the can opener. I know, I know, you were thinking I was gonna say the microwave, ya know, for popcorn and making marshmallow Peeps explode and shit like that. But no matter how you slice it without a can opener you are gonna have a hard time getting your can of Chef Boyardee ravioli open. God knows how they get that shit into the can to begin with. It's probably a conspiracy from the can opener lobby so that they can keep us bent over a barrel when it comes time to chow down on some hangover food, the bastards.
Then again, the invisible fence, ya know, for keeping dogs in the yard, is a pretty clever invention. Don't get me wrong, I love dogs, but I don't love getting mauled by them if ya know what I mean. Granted, the majority of my canine incisor interaction has been at the paws of police dogs just doing their job, so I can't really blame them. But as they say, once bitten twice shy, blah blah blah.
Of course mention must be made of the invention of candy raisins. Ooh! And also watermelon Jolly Ranchers! Green apple flavor too! And remember when wax whistles were really big? Yeah, that was some cool shit.
Mind you, this is all coming from an American perspective. No doubt you folks in Europe would top your list of greatest inventions with something like the "flesh light" cuz, well, you're European.