I think I'd go with a more well balanced global economy. If every country was competing at the level of the top countries I think everyone would benefit greatly, and it would snowball into solving even more problems down the road.
Fast quantum computing in the order of many gigaquibits per second or so.
After the breakdown of all previous crypto being basically obsolete then medicine*, materials, hard sums limiting all manner of physics, chemistry, economics... solved essentially overnight. The relative jump in tech that would result in that would be like dragging a caveman that just figured out how to stick a skin up in front of the cave opening to the modern world.
*all medicine today is so much "this worked in the past so what if tweaked slightly so" and "what does this button do". This would allow you to basically trivially simulate the interactions at atomic level of chemicals and backwards engineer solutions for problems. Same for most chemistry, materials (if I add 1% by weight of this to this substance then what changes, now 1% of this... there are several hundred elements and the relative change in proportions and interactions mean even our most fancy stuff today is but a handful of them. You could make jet engines use a fraction of the fuel they do overnight if you can make something that takes the heat, and no reason to believe you could not do it.
Though as that is more sci fi and not problem.
Would sort out education. It has been known to be fundamentally broken for about a century now (I have books from the 30s that could have been written yesterday).
Sort that and a lot of problems basically vanish overnight.
Tough question. That is: I've got two top answers that...probably play into each other (hard to say because this is pure theoretical, of course). That is: economic inequality and global warming.
Economic inequality still plays a bit of an underdog role. That is: we KNOW the average Joe doesn't have as much income (let alone disposable income) as your average millionaire, and his wealth isn't even up to snuff to what the 0.1% have. Shit's exponentially...and even that's still only taking the known capital into account.
The whole reason inequality is a bad idea isn't so much because it's convenient (I'm part of the 99%, so of course I'm rooting for it), but more in that THE VERY RICH CAN'T SPEND IT WELL ENOUGH. There's probably plenty of holes you can shoot into trickle down economic theory, but this really is like a size elephant cannonball hit in the critical part. Because of this, the inequality keeps on growing and more and more capital becomes "sleeping money" while the rest of the world is struggling more and more to get by.
Somehow fairly distributing it (whoo, commusocialistic anarchy!!! ) would reduce or even remove things like worldwide stress, suicidal rates, inhouse violence, child neglect, long illnesses and a whole other slew of bad stuff of our time.
...the thing is: we're still bred to be consumers. How will global supply chains look like when the manufacturers's employees aren't treated like dirt anymore (which they will no longer do because said employees can just decide to not come to work anymore) ? They'll have to increase cost, which increases prices, which...and so on. I'm inclined to say that this sort of problem will return by design, so "a solution" can't just be a one time miracle but has to work continuous to be maintainable (or humans need to be re-educated...or become extinct, which is probably the easier solution).
And the main flaw: where's the ecology in all this? If 99% of the population gets more buying power, will the environment somehow actually benefit? Erm...I kind of doubt it, to be honest.
Global warming is getting more urgent each year (or each month, it sometimes seem). It's like only after we've past the point of no return the lobbyists denying its existence shrug their shoulders and leave us to fix the mess...on their conditions.
Yeah...that certainly won't work. We're still currently operating under the thought process that got us into it (it's called capitalism(1) ), so solutions posed on that front won't get us out.
Meaning: solving the problem means embracing a different kind of ideology. I'm all in favor for that, because...well...just read the previous chapters here.
Hmmm...now that I've written it, I think solving this problem also at least reduces inequality (hint: things are currently so unequal, the majority of earth simply doesn't even have the resources anymore to make a kind of worldwide change -> the solution MUST come from the upper echelons). So...
for my pick, I'll vote to change global warming. As in: right the fuck now.
(1): to be fair, most if not no other political ideologies have ecology built into them by design. It's just that capitalism is currently leading the world, and therefore gets the blaim
Curious how many people rank climate change as a major and seriously pressing threat to have it as a primary one.
As far as economic inequality then from where I sit that depends what the desired result would look like to people. I also wonder what goes when people do all end up fat and happy as it were (see also a lot of welfare fun and lack of drive there, and the results of big inheritances/lottery wins on people that get them). Incentives, hierarchies, capital limitations on excess, and drives are very much a real thing and eliminating that... results not necessarily pretty.
That said in as much as nobody is super struggling for reasons other than their own serious stupidity. That I would be up for solving as it were. If it could be swung such that people can go in for more esoteric stuff rather than listen to the siren song of boring finance and whatnot jobs that would be possibly preferable for me; too many times now I have met amazingly talented people working finance jobs they hate far outside still very useful fields that they would otherwise be doing for.
Those that would want to go further and tear others down... that is where Mr Rifle comes out to play and I can't see that ending pretty. It would not even be that useful; total up all the "wealth" of various fractions and even if you could magically realise it (and it was not tied up in stuff you can't sell lest the price tank) then you are not going to be able to fund too much for long, and the system itself relies on stored reserves of various flavours to offset the risk of trying new things, new things being the only way of going forward
It would not be very pretty but I imagine some fun with chemistry. Various fungus and plankton could probably be treated to do some things that would more that cater for proteins, vitamins and carbohydrates and in locales not disposed to much.