Wunderlist manages your tasks with a few focused features for organization, all within a beautiful interface and at no cost. There are not many features to speak of in Wunderlist, but its minimalist focus is what makes it work so nicely. Aside from being really pretty, the interface is straightforward and easy to use. You can create lists, add tasks, decide when they're due, and search and sort through everything you add. All your tasks and lists are synchronized with your wunderlist account. It's free on all platforms, fast, lightweight, easy to use, available on the web, and syncs across all of your devices.
Besides offering the latest multi-touch gestures for task editing and management, Astrid has all the goodies like reminders and notifications, syncing to the Astrid webapp and shared tasks with other Astrid users on iOS and Android. Astrid allows you to track your to-dos and projects, set due dates and deadlines for your items, and be alerted when they're due. You can organize your tasks into lists and categories, and then sync those tasks with the Astrid web service or Google Tasks.
Orchestra is designed to be a to-do list app that works kind of like email, letting your tasks live outside of the app and interact with others. It'll let you send tasks to other people, whether or not they use Orchestra, and they can request your assistance on anything as well. You don't need to collaborate to use Orchestra, however, as it's still an excellent to-do app regardless of what features you use. One of the places it shines the most is in the many ways it lets you add tasks. You can forward emails, speak to the app, or type them in normally. It also offers plenty of task viewing filters and methods of organization. It's simple and easy to use, too. It is, overall, about the best way you can manage your tasks because it's flexible enough to work the way you want.
With Any.DO, you get the benefit of multi-touch gestures to mark tasks as complete, the ability to start typing a task and have Any.DO autocomplete it for you, Google Task syncing, and the ability to add to-dos by tapping the microphone icon and speaking to your phone. The webapp on the other hand is just as intuitive as the mobile version, and Chrome users can install the Any.DO Chrome extension to manage their to-dos without having to keep an app tab open. Additionally, sharing is huge in Any.DO, and you can share your to-dos with Facebook, Twitter, and more. The webapp, iOS app, and overall service are still free, and they all sync up nicely so you have the same to-dos on any device.