Surprised many people here seem to think Google Drive is the defining competitor. When the real money is made in enterprise, which I would say Microsoft dominates with OneDrive. Microsoft's bundling of Office 365 with OneDrive and internal sharing is something only Google could compete with (but can't because at least for large corporations, Google Docs does not suffice.)

I have found that Google has the best for collaboration, but the money is made in enterprise and OneDrive is making in-roads because of what it is bundled with.