Obsidian Sync is a good service (at least, I've always found it very fast and reliable; pace Paul!), but it's relatively expensive for just one vault and one gigabyte of data -- and twice the price for multiple vaults and 10 GB of data.

Enter a brand new service called Syncstone, based in Switzerland with servers in Holland (the Netherlands), which charges about the same for two gigabytes of data and any number of vaults. Furthermore, Syncstone supports collaboration (on the same note!), offers end-to-end encryption (E2EE), and allows you to access your vault via a browser. So far (and I've only just started testing it), the syncing appears to be pretty much instantaneous.

The website is at https://syncst.one/

Worth a look, for those who are heavily into Obsidian! ;-)