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