The most important problem is bettering oneself. Aligning your subconscious with your rational mind and making sure that you don't do stupid human things like greed, pride, jealousy, wrath, etc.

Almost everyone thinks they are a "good person" and "don't do these things" but almost everyone has certain situations that trigger them or behavioral patterns that automatically include them, i.e. almost no one is actually as good as they think they are.

Identifying these triggers and patterns in you and taking steps to mitigate/remove them is the largest impact you can have on the world.

It's also one of the hardest things because admitting your wrongs to yourself is very painful. One's ego is a powerful thing that is one of the worst pains a human can feel when bruised. That's why it's so much easier to look for "other people to blame", it keeps your ego from hurting you.

But each and every person thinks like that and thus becomes "part of the problem". Even if you're a "better person than X other person", that doesn't mean you should just stop improving yourself and try to change X instead, because chances are, you'll never change how X behaves. But it's much more likely you'll be able to change how you behave yourself and actually have an impact on the world.

If you try it for a while, or if you experienced it yourself, it's very ineffective to try to change other people. The only thing that really works is helping people change themselves. But if the will is not already there, there's basically nothing you can do. And thus, creating that will in yourself is the best thing you can do.