The problem with every app like this is the lack of good quality material. For language learning, I prefer learning whole sentences as examples of an appropriate context. But creating good lists of sentences & translations is cumbersome and just throwing a tool at the "crowd" doesn't automatically get you quality content.
That said from all these tools I prefer anki since it has a web-based UI and an android app.
That said from all these tools I prefer anki since it has a web-based UI and an android app.