If you're in the Apple ecosystem, I recommend iCloud for syncing Obsidian. For one thing, iCloud Drive gives you 5GB free, which — as long as you're not using it for photos and the like — is plenty to get started with at no cost.
To set it up, choose iCloud when you create the vault on your iPhone or iPad; on a MacBook, just open the vault you created from the iCloud Drive/Obsidian folder.
There is, however, one more thing you need to configure. Let me explain that part.
Keep downloaded
There's a reason Obsidian is slow the first time you launch it on the iPhone (that moment after the logo when the loading bar appears). When Obsidian first launches, it loads every file in the vault. That's the only way to get the connectivity and fast search that Obsidian prides itself on.
The thing is, iCloud Drive removes files you haven't used in a while. So on that first launch, those purged files have to be downloaded all over again. This is one of the biggest reasons Obsidian is slow to start on the iPhone.
The fix is the Keep downloaded feature introduced in iOS 18. With it, once a file has been downloaded it won't be purged even if you don't use it. As in the screenshot below, select your Obsidian vault in the Files app and turn on Keep downloaded.
The same goes for macOS and iPadOS — enable Keep Downloaded there as well for a smoother syncing experience.