Best cloud storage (tie): Dropbox
Apple's iCloud is a great service for keeping files and other data synced across iOS and OS X devices, but it's not (yet) a storage service where you can keep files in a central location accessible to all devices and other users. Dropbox is such a service, and it's integrated with many iPad apps, so it can fill in as a common file system in some cases. Dropbox also integrates nicely with OS X and Windows, appearing as another storage volume. It's available for Android as well. Note that using Dropbox with Apple's iWork apps requires a $5 monthly fee.
Price: Free, but charges for storage vary
Compatibility: iPad and iPhone