My vote goes for Syncthing too, simple single binary install and works with no configuration. I've installed it on my desktop, vps, laptop, and phone use it instead of Dropbox now.
I've even got two non techy friends to install it (albeit talking them through it) and it still works to this day.
I've even got two non techy friends to install it (albeit talking them through it) and it still works to this day.