Tsumego Pro (life-and-death) for Android
Expensive and UI needs improvement along with more ko problems. The Google sign in prompt should also be dropped along with the cross hairs when placing a stone. At least make the cross hairs optional. Decent selection of problems, just needs more. Marked down to one star as it doesn't even keep track of where you left off.
by X####:
Really like that problems are sorted by topic and difficulty, not just difficulty like the other Tsumego Pro. The UI just isn't that great. Price is steep, but a lot cheaper than a book of problems. And like I said, you can focus on one problems on one subject to get better at that subject. That's why I use this app.