Free. Current App Store rating: 4.5 stars
There are many worthwhile to-do list iOS apps, but Wunderlist is one of the best. It's simple to use, has an attractive interface, and gives you lots of organizational features. You can add subtasks, a helpful feature not every to-do list manager offers; add notes; attach a file, such as a photo or video; create reminders; and more. The app's advanced features, including the ability to assign tasks to collaborators, cost $5 per month or $50 annually. Other worthwhile to-do list apps include Any.DO (free) and Pocket Lists ($5).