Personally, I look for three things in a GTD app — simplicity, syncing, and badges. 6 Wunderkinder’s wunderlist has the right combination of design and implementation that works for me, and it’s free to boot.
wunderlist is elegant. To-do items are separated into lists, and adding an item is as simple as tapping at the top of a list page and entering text. Nothing else is required. Due dates and notes can be added if desired. The touch-interactive list view of items is extremely intuitive with support for priority tagging, drag-and-drop reordering, deleting, and marking as done. One complaint regarding simplicity is that the user must go into a list to add a to-do. Top-level (unlisted) items would be a welcome addition.
6 Wunderkinder is committed to keeping wunderlist for iOS (as well as its desktop counterparts and accompanying syncing service) free. Registering for syncing is simple and painless, and can be done in-app. Like most iOS apps that sync over the air, syncing occurs when the app is loaded, not in the background.
Badges? Badges?? I need stinking badges! I have the same problem most people have with GTD apps, and that is actually GingTD. As an artist, I can admit that if that red badge of urgency isn’t mucking up the clean lines on my home screen, I’m not going to remember to see what’s on the to-do list for today. The app badge indicates items that are due today or overdue. As with syncing, badge updating happens when the app is active (no push notifications).
In a world of a million GTD apps, it often comes down to personal taste and working style. For my needs, this clean little app does the trick. Bottom-line, wunderlist ist wunderbar!