Dropbox is in my experience the easiest to use; it is easy to upload and share documents across mobile devices. That being said it is more consumer focused than business oriented, although they are making efforts to beef up their business offering. Box.com is more enterprise oriented - you can easily set up data rooms among internal and external teams of users; however I find Box's usability across different devices (laptops, iphone/pad, etc.) to be more difficult and cumbersome. My company is currently beta testing enterprise box. Both dropbox and box are easily scalable based on number of users. My understanding from my IT department is that Skydrive/OneDrive does not have the level of encryption and security that box and dropbox (and other cloud services) have.