OneDrive

OneDrive's signature trick lives in File Explorer, not a browser. Files On-Demand renders cloud-only items with a tiny cloud glyph; double-click and they hydrate, with status icons distinguishing online-only, locally-available, and always-keep states. It's among the tightest OS integrations any sync service offers, because Microsoft owns the shell. That intimacy is also the failure mode: OneDrive injects itself into save dialogs, claims the Desktop and Documents folders, and produces sync conflicts whose error states stay notoriously opaque. The web client is competent but secondary; the real interface is the file system itself. When it works, you forget it exists. When it conflicts, you get duplicate filenames and a notification that explains nothing actionable.