I heart kanjibox
Great value for money. Huge range of vocabulary with very helpful usage examples. Very customizable. My favorite thing is how the levels are organized (by JLPT test level) but in a way so it is possible to study previous levels and even sometimes have some unknown words thrown in. I also really enjoy the ability to compete in quiz mode. Very impressive design. I havent bought a lot of the more expensive Japanese learning apps but I think they have a hard time competing with the value for money you get from kanjibox.
MojoMcjojo about
KanjiBox, v2.7.1