How could you best spent one million dollars, to materially help the world in a lasting way?

1 million dollars doesn't go that far these days, at the world scale it is almost nothing.
You could however make a lasting difference in your community by making a scholarship, building needed facilities, or doing something else where you directly make sure the funds go where needed and can't be used for other uses. Lets face it, if you give it to some charities 90% of it will just go to the administration.
- One of the things that I've seen is that politicians can be bribed for shockingly little money. I think with a 1million dollars you could bribe one high member of congress, or 10-20 low-level politicians.

Follow in the footsteps of Gil Cisneros...
"...is an American government official, philanthropist, and politician who served as Under Secretary of Defense for Personnel and Readiness in the Biden administration. He served as the U.S. representative for California's 39th congressional district from 2019 to 2021
"In 2010, he and his wife won a $266 million Mega Millions lottery jackpot and became philanthropists, establishing endowments for scholarships to be given to Latino students at GWU and the University of Southern California. They also founded Generation First Degree Pico Rivera, with the goal of ensuring every Latino household in Pico Rivera has at least one college graduate, and the Gilbert and Jacki Cisneros Foundation with an initial investment of $20 million to provide mentorship in education."

Hire as many hitmen as I can to kill the wealthiest people. But we would have to keep doing it, and it wouldn't dissuade anyone from hording money...so?
- Put 3/4 on the hit and 1/4 betting on the stock market reaction only you know is coming. Profit. Repeat.
- I doubt professional hitmen would go after the real fat porkys for just a million.
- Well, every time assets get split between their 3 kids, you reduce the assets accumulated in one couple.
- So you'd choose senseless violence. Good thing you're not getting any money.

Give it to me.

Dedicate the $1 million to funding zoning reform efforts in your city to allow for the construction of more housing and reduce the cost of housing for everyone.

I have an idee fixe that I could set up a non profit that bought homes and rented them at a price somewhere between the maintenance cost and the market price. It would make a profit and slowly expand providing more and more affordable housing. Ideally it would start with more than 1 million but doesn't need to.
- If you rent at cost or even a little over you'll go broke. People won't pay their rent and trash the places. Non-profits are finding out that they can't even support what they're trying to do with housing because the actual costs are more than what you think are "at cost".
  
  I think that's why they said between the maintenance cost and market price.

Set up a trust to invest the money, from the compounding returns, use a sustainable portion of the dividends to fund a cause you believe will help change the world. Be very explicit to the trust managers as to the long term instructions of the trust.
With compound market returns the million dollars could be worth $40 million in 50 years and would be distributing af a sustainable 3% dividend, 1.2 million a year to a cause you believe in.

Guillotines with "Heroes Work Here" signs atop them.

I think spending all of it on feeding people who are starving. You could keep a lot of people alive on a million, even if it was just for long enough to give them a second chance on their own. And even if that second chance failed, then at least all of the goodness of those people would have more time to be in the world.

Give water and food to where it is needed.

You might be able to buy some land, build a playground and maintain it for a few years in a deprived neighbourhood. If you have money left over, do it again somewhere else.

- I'm generally against the idea of planting as many trees as possible.
  Trees are not very good carbon sinks because they decompose and burn. Also, there are also some ecological communities where adding trees makes the land a worse carbon sink.
  Avoiding cutting down forests to build suburbs is something I can certainly get behind though.

Fund an open source project to make something useful that helps make life cheaper and easier for regular people

Free education
- For like 5 people?
  
  Then I suppose the question is reduced to how one should select the 5 people! :)
  
  Anything better than 0
- Well, if you put that million dollars somewhere that makes a bit of money and follow the 4% rule, you could take out $40k each year and maybe help 4 kids attend a state school every 4 years.

According to Rutger Bregman: just give it away to people who don't have it.

Call up Louis Rossmann and have a talk about how you can help with democracy to stop the present dystopian neo feudal regression in the world. Long term, you could impact the trajectory of the next few centuries in substantial ways and lessen the coming dark age.
- Long term, you could impact the trajectory of the next few centuries in substantial ways and lessen the coming dark age
  With a budget of $1,000,000? That seems very ambitious at best.
  
  Not really. There are very few lobbyists for a non dystopian future. The battle for the right to own your tools is the absolute fulcrum of the future and the next several centuries. The loss of ownership rights is the largest sociopolitical issue and regression of the past millennia. The atrocity of feudalism was already hashed out as a terrible and failed social structure. Allowing it to reemerge will have extreme long term impacts
  The way people fail to see and understand this issue speaks to the potential force needed to shift the trend and trajectory. All it takes are a few influential and connected people working to shift the political conversation and momentum in the opposite direction to alter the course of the future. Funding a few individuals to speak up for us could make an enormous impact. Ownership IS citizenship; IS democracy. Trusting others while renting tools and property IS feudalism. It is a path to slavery in all but name. It happened before, and is always the inevitable outcome of this situation. Putting up any fight against the lackadaisical complacency of our present culture absolutely has the potential to impact the future in a substantial way.