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...
Temboo supports a number of cloud storage APIs, and can generate the Java source code you need to access them.
See here: https://live.temboo.com/library/keyword/storage/
By normalizing API access, Temboo makes talking to one API as easy as talking to the next, so it sounds like something that you'll find useful for this project.
Full disclosure: I work at Temboo.