To glibly answer your question: favorite way? In the "usercloud" (as a cookie).

But more in depth, it's a good question!

You could get innovative and use localstorage combined with WebRTC to create a p2p storage "mesh". :)

Honestly tho I would use GitHub gists. They have an API. There's a token you can get. You could use whatever storage format you want (JSON anyone? Blah, why not just use malbolge).

Or if you're doing gists, why not go the whole nine gits? Depending on what your game runs on, you could just use git, to a shared repository, on GitHub.

A gist is a great idea. The one issue, I think, is that I cannot expect users to have a GH account, and I'm not sure it's sensible to make a GH account and embed a Token in the app.

That's the rub, I think. Any sensible person would scream, "SECURITY HOLE!" but for my use case, it's not an issue. Just some high scores for a game that like 100 people might play in its lifetime, or whatnot.

Encode your game state to the URL itself and tell the user to bookmark that URL. User can then use browser sync, email or whatever to sync the bookmark to another computer to continue where they left off. No services, no tokens, no sharing data to some random company or person.

A URL is about 2000 characters max, so this definitely works. It's a more portable than localstorage given they can just handle the URL themselves.

This doesn't cover collective access. Eg. high scores, though. I'm still thinking on that one. I think it's going to be impossible to find because of security implications. Services are either going to not want to make it work, or require accounts.

I used a free redis instance. There are all sorts of free database and storage services out there. Cloudflare has some really cool free stuff.