Best cloud storage (tie): Box
Like Dropbox, Box enjoys wide support from iPad apps, and it works in Windows, OS X, and Android. It now offers the simple integration with OS X and Windows that Dropbox long has boasted, making it easy to work with files. Box can be used with Apple's iWork suite on the iPad for no additional charge, and it offers an enterprise version that lets IT manage access and content rights for corporate data.
Price: Free, but charges for storage vary
Compatibility: iPad and iPhone