Ask HN: GTD/Checklist apps for iOS or Android

Long story short: I tried a few simple (checklist) and advanced (GTD) apps and was underwhelmed. I considered writing my own app, but before doing so I should ask: a) Do you use any gtd or checklist apps on iOS or android? If so, which? (looking for an iPhone/iPad app, but I'm curious about those for android as well) b) Are there any features you would like to see in a GTD/checklist app?