The Most Useless(ful) Thing
Speaking only of matters regarding the human race, this answer must be obvious. Oughtn't the most useless thing be that thing which spreads around the most uselessness? Thereby, the more effectively a thing creates uselessness in its individual users, the more effectively that thing spreads uselessness into the human world. It is the most useless thing based on its capacity to disseminate the largest collective engagement of human uselessness.
So what is this yet-unnamed useless thing? The current mad dash human progress is engaging in to gobble up every little tendril of useful behavior and relegate its responsibility to the functioning of a thing is creating a rabid marketplace for things with increasingly improving abilities to take responsibility for human behaviors, thereby creating larger margins of uselessness in the time and activity of humans. (Humans love this, of course. Free time lets humans do really useful things, like respond to unpaid writing challenges.)
This all also creates a market for these things (which is not necessary in a survival sense) that strongly reinforces the perceived need of said things, and a related degree of capitalistically-relevant urgency. Is it computers? Is it mobile computing technology? Perhaps microprocessors and semiconductors that promote exponential growth in sophisticated technologies around the world? Could it be all of these things, all together, as one lump-sum-of-a-victim in this vicious blame game, the wretched idea of technology itself that has decimated humanity's ability to think for itself?
Or...
The most useless thing is something else. The most useless thing is that which, at the moment of observation, is doing the greatest amount less than it could be doing if operating at 100% efficiency, the greatest differential sacrifice of its total potential use.
...Oh, C'mon... Don't make me say it...
Actually, it's not humanity's fault. The ego is an incredible psychological construct that is phenomenally helpful in allowing people to navigate their lives and keep waking up every day, even if they don't know whether or not there is any point in that; it can't be the ego that is the most useless. And the ego can't be blamed for not wanting to participate in this very, very specific and particular game of finger-pointing. But if you look closely enough, right over to where the ego is very firmly not pointing it's finger, perhaps you will see what is hiding there in plain sight, emanating its uselessness into the world in unmitigated and unrelenting torrents. It could be right there in the room with you, right now, reading these words right along with you. Or maybe, just maybe, it isn't... for now.