Pocket Informant is an amazing application. I've been using it since it first came out on the iPhone. Prior to the iPhone I used it on a Windows Mobile device. The iOS version has grown up quite a bit over the years and it is now one amazing app.
Having been an avid user of the Franklin Planner, Pocket Informant is the only iOS app that can use the Franklin Planner methodology (A1,A2, B1, B2, C1 C2) for task prioritization. It also has the capability of doing GTD as well.
The latest version also has Evernote integration
If you want a really good app that will combine calendar, tasks, contacts, notes then get this app.