There is a saying in Hinduism.
What is the greatest wonder?
That death comes every day, yet man lives as though he were immortal.

Critical thinking. It would solve a lot of the world's problems.

How to be empathetic-Caring about issues leads to doing something to address them-protest, rebellion, not standing for the status quo.

Treat others with full dignity and respect.

Being Love and ditching fear - being unafraid to be love/loved.

Warning: As usual, my reply is very long. But I like "thought experiments". Indulge me, if you please.

"...what concept would have the biggest positive impact on humanity..."

I have no answers. I tried to formulate a logical solution to, what I think are, the two main causes of all the problems we have experienced since we became sentient.

I think those two main causes are:
Jealousy: "those guys have more stuff, let us take those stuff for ourselves."
Selfishness: "no way we'll share our stuff with them. we need to look after ourselves."

Are there other fundamental human traits you think cause the big problems we face as a species?
Anyway, my reply will focus on these two traits.

To continue...the solutions I came up with to stop jealousy and selfishness always hit on one flaw that is inherent to us humans: we have evolved to fight for ourselves, our families, our communities, our countries. we have not evolved to fight for others.

Sure, our current level of consciousness and understanding of...everything we know right now...have tempered a good portion of humanity to be more altruistic. And our societies are a testament to that.

HOWEVER, there are still a good portion of humanity who are both jealous and selfish, right?

How do we solve THAT problem with one concept and that would have the biggest impact on solving that problem?

The HIVE MIND.

What that is and how that would work or how to get the different peoples to understand that concept and to allow it to happen is another question. This is just a concept after all.

I'm not talking about being non-feeling robots that only consume what we find.

I'm talking about the concept of a physical thing (maybe a super computer or whatever that super computer may evolve into)...a physical thing that would control what we do. Examples of what this thing would do are:

E.g.: Someone over there is unhappy. Find one of their good friends and send them to entertain or listen to their problems.

E.g.: That person there is not suited for working outdoors because of health issues. Replace them with someone with a stronger constitution.

E.g.: There's a single person looking for a partner. Let's find the most compatible person for them and make them meet. If that relationship doesn't work, lets find other people for them.

E.g. That person has passed their engineering test with flying colours. Get them to work for that engineering company.

E.g. That person has lost interest in doing his job. Look for another job he would not only enjoy but would develop him as a person further.

One limitation this super computer (or whatever this thing will be) must have is: It shouldn't control our emotions. It should only determine what we need to do for the betterment of others - everyone.

The HIVE MIND.

To tolerate: we know everyone thinks different and we do not agree on the conclusions reached. However there needs to be a point of understanding to learn from a person's point of view.

That Anthropogenic Global Warming is real and a serious threat to the state of equilibrium on the planet.

Critical thinking

Everyone should be treated the way they wish to be treated, not the way I want.

Live and let live.

Every human should do LSD once.

Just kidding, kinda....I would say love and forgiveness.

I would at least try to teach them that their belief or opinion (be it theological or otherwise) is not necessarily better than the equally dearly held belief of others. That unshakable belief in superiority shuts down any sort of chance conversation or compromise.

Do what u expect from others..

You're a person.
You aren't any better or different than anyone else.
I don't care about who you fuck, how you identify your fucking, what your skin color is, what your monetary value is, or who your imaginary person in the sky is.
You're a damn person.
Stop with the bullshit semantics
And get over yourself.
Let's save the planet,
Terra-form Mars,
And get rid of the white man by interracially reproducing.
????

How about peace in the world?

Hold their breathe , no more people to destroy the world ......lol sorry couldn’t resist

Compassion

Believe in yourself. Have faith that the love you have for you will emanate, and bring out the love in others. No hocus pocus magic of prayer of invisible friends. You entire foundation of belief starts with you

Compassion.

Treating their mind like a sacred tool, which would be to critical think and question everything to protect it from manipulation.

All we really have is each other. Play nice.

Compassion, but someone beat me to it! lol

Critical Thinking/No Idea is sacred

Keynesian Economic Theory. The world needs job creation programs.