I need to create a Java web app that uses the API of at least two different cloud storage providers (Google Drive, Dropbox, SkyDrive, Mega, ...). I'm wondering if there's someone with experience using these APIs who can tell which are the easiest to use and which are the most difficult...
Kloudless provides a common API to several different cloud storage APIs (Dropbox, Box, GDrive, OneDrive, etc.). Kloudless also provides SDKs in popular languages and UI widgets to handle authentication and other user interactions.
You can find more information and sign up here: https://developers.kloudless.com/
Full disclosure: I work at Kloudless.