I'd make a development fund and fund renewable tech, green startups, lobby politicians for policy changes like a carbon tax to pay for social programs.

I think it's best to use the fund more or less like Bill Gates did.

I'm going to fund and build a ton of mass transit projects.

Burn it. Gas and Dynamite are cheap.

Raise my own private army

take over governments, be a benevolent dictator who forces people to lean that dictatorships are bad, that forces everyone to go to school (free education) and get a good minimum amount of basic knowledge

put a strictly controlled capitalist system in place that can make lots of money that will go to a socialist system that makes sure everyone can be happy.

force free healthcare (mental, dental, eye, all of it), free basic housing, decriminalize all drug, but school everyone in responsible recreational drug use and setup treatment centers for those that fall off the wagon.

Restrict free speech to the point where it makes sense. Everyone can talk whatever they want but if you have even a small following you better stop talking non scientific non democratic bullshit or I'll jail your ass

Put in a boat load of restrictions in politics. All money must be visible, period. Politicians cannot own companies, they ca not own shares. Politicians will not be allowed to make decisions that step into things that are good for them personally. Remove political parties, all politicians should be independent, no more party politics. Voting would be per post. All politicians can register themselves for a post, show why they could do it best, and then we vote for the posts. Voting would be a Nation holiday and voting would be obligatory for at least 50% of all available posts

Get rid of churches and imaginary bullshit. Humanity has entered the era where it needs to grow up and stop believing in fairytales. If this means that we won't get good horror movies anymore then I can veryuch live with that.

On capitalism: put a hard limit on howuch someone can own and owe. Debts would be limited to just a few thousand dollars per person. You can't ownore than 10 million, anything beyond that will go to 100% taxes automatically. This will vaporize the gap between rich and poor.

Nonmoree private planes, mega yachts, etc. if you want to have a house & boat, as long as it's under 10 million, go nuts!

Free minimum housing, make basic food items free, education free, healthcare free.

Switch to a 4/3 work week

Companies will he taken out of politics, they are not a person, they don't have free speech, they won't have speech. Company sizes will be limited to 5000-10000 employees, that's it. Beyond that, it will be split up automatically. Companies will have to work together on bigger and mega projects

Justice systems would be reformed, now focussing on keeping the innocent out, instead of the guilty in. Focus would be 95% on rehabilitation, education, self improvement, the rest on the punishment part.

Break up and where possible, remove banks, and make them as small as possible

Put extreme restrictions in what can be speculated on, and invested in. You can't buy debts, you can't bet on companies failing, you can just invest to try and help a company a little and get a bit of a reward for that.

Ban all discrimination under heavy penalty. Live and let live.

Guns and weapons would be available under very heavy restrictions for sports, hobby, etc.

Marketing would be severely restricted. Companies can't lie anymore, period. Commercials would be severely restricted. Nothing towards or for kids, products can just look what they are.

On the environment: a huge push will be made to transform car dependent cities to human dependent cities. Roads will be changed to parks. Cars will be allowed but cities will be humans first and it'll be easier to get around with public transportation or bike or walking than by car. This will immensely change everything. Humans will be healthier, everything will be quieter and more beautiful. Architecture will change as giant depressing car parkings will change into beautiful parks. People still can have a car if they need to but most won't want to (see the Netherlands for the past 4 decades, and France where they recently started this too)

Once it's all rolling smoothly, I will then step down, and start the elections, and I'll move on to the next country to invade and take over to deploy my evil plans. Heeeeehehehehheheeh.

Start building nuclear power plants for a start. Maybe spend half of the 300 on that.

After that, start buying large chunks of land and rewilding them. Old factories, plants, warehouses, that sort of thing. Make deals with local authorities so I can donate them and they'd stay rewilded.

If I can't make those deals work, well, that's what lobbying is for.

300 billion, secure myself first, then build up companies in my interest, then use that capital to influence politics, then secure policies that will help my personal and business objectives, then create a charity that will have a tax exemption for planting trees and r&d a biodegradable, easy to manufacture, durable, plastic that will replace all current non-biodegrable plastics. Wouldn't become prime minster because you can get the most done behind closed doors. Also need to use some of the money to secure my kids future.

A question for the economist here, does paying my country's national debt help?

Retire to some island in the middle of nowhere and build myself an evil villain lair. Once money runs out threaten for 1 Beeeellion dollars. If 300 billion usd isn't spent then donate the rest to some child cancer hospital or something anonymously post-mortem.

Any other choice would be suboptimal.

Donate to cancer research

Create the world's largest charitable foundation and make everyone beg before their presentations.

Nuclear Warfare

300billion of what?

For the low-hanging fruit answer: Invest it. Too many things to do for 300 billion to be enough.

burn em

Eradicate cars, have reliable public transit that runs 24/7 and departs every 5 minutes. All deliveries are via helicopter for businesses and drones for personal orders. Turn parking lots into something actually useful. Make all fast food places completely automated and open 24 hours. Eradicate homelessness and poverty.