I've had a lot of success with a combination of Skritter and the Chairman's Bao (TCB).

Skritter has been my favorite vocabulary building app. For a bunch of reasons:

1. Supports SRS. It makes study time very efficient by making you practice the stuff your bad at rather than stroking your ego by asking you questions you're good at. The only downside is that it doesn't stroke your ego for you you'll need to do that yourself by noticing how fast your vocabulary is increasing.
2. Great writing practice. It recognizes strokes, in the proper order and corrects you when you're wrong.
3. A rich set of company and user generated "decks" (curated vocabulary lists) and the ability to create custom decks.
4. Automatic deck creation. You can integrate with other apps (like the TCB) to reinforce your learning.
5. The developers are a small team but they're fairly responsive.

The Chairman's Bao is a graded news reader. They create a bunch of articles based on vocabulary from different HSK levels. They have a nice integrated dictionary that will tell you about specific words in the context of that text. They also let you export those words to your Skritter lists.