Apart from those that have only appeared in a lab for a few seconds what element is the most useless, having no notable biological industrial or societal function?
Dubnium.
Francium.
Politicium. Totally inert except when exposed to Richbastardium.
I suggest looking at the commodity market, those with the highest prices are the ones with the most use compared to the available supply.
So is the OP excluding all man-made elements or just the unstable ones?
An interesting side question would be what is the most useless element with the lowest atomic number? Boron?
But on that criterion elements like carbon, oxygen and nitrogen have little use. However, since they are essential to life, they hardly satisfy the OP.
And the most useless element wouldn’t even be a commodity, being of so little use.
Also, as said, it’s uses relative to supply. Iron is dirt cheap while still being one of the most useful elements.
My first thought was one of the Noble Gasses, but some of them have use as fire suppression.
Boron is used in the form of boric acid in buffered solutions and HPLC mobile phases. So, I don’t think that qualifies.
After a brief google query, I would submit that Protactinium would be a good candidate. According to wikipedia, it has virtually no uses outside of scientific research. With a half-life of 32,000 years, it also lives long enough to qualify.
There are many industrial and scientific uses for noble gases. Neon is used in neon lights (neon signs), helium-neon lasers, hollow cathode lamps, etc. Xenon is used in HID lamps (headlights in high-end cars) and halogen lamps. In my field of research it’s used in X-ray detectors, extreme-UV generators, and also as rocket fuel (propellant for ion engines). Argon is used in industrial processes where you need a non-reactive gas, such as TIG welding (which stands for tungsten inert gas welding).
I admit I didn’t know of any uses for krypton and radon off the top of my head, but Wikipedia lists many uses for them.
I think protactinium is a good one. Basically all of the transition metals are useful somehow, astatine and francium don’t meet the OP, and the lanthanides may not have been useful 100 years ago but they sure are now. Heck, most of the actinides would qualify, assuming they have a half-life long enough to suit the OP and depending on what counts as useful.
Well, duh, defeating the Man of Steel!
Sheesh!
That was kryptonite, not krypton.
Beryllium might be the least used of the really low atomic weights. (which I distinguish from least useful)
This is because of the risk of berylliosis.
I recall a proposal to use beryllia at a former employer as an insulator for some advanced electronics. The safety issue was raised, both in the risk in manufacturing it, and in the inevitable disposal of the product at the end of it’s life, or through accident.
The least useful rare earth would be an interesting thread . . .
Boron may have its uses, but it is surely the least interesting element. 
True, but beryllium is invaluable for certain specialized applications. Because of the low atomic weight, it’s pretty much transparent to X-rays, so it’s widely used as exit windows for X-ray generator tubes. Some types of X-ray detectors use them as well.
It’s also used in some aerospace applications where the light weight and thermal properties (low thermal expansion, high melting point, etc) justify the handling difficulties. The mirrors for the James Webb Space Telescope, for example.
I note beryllium is used in tweeters (of all things), it being a matter of pride for some audio aficionados to have them in their home theater systems. Beryllium is useful, no doubt about it, but it can also be a deadly poison too. IIRC, early florescent lights used it, and there were some medical tragedies as a result.
I vote for Lutetium.
Also used in drills and tunnelling machinery.