I'm a big fan of OmniFocus. I've been using the Mac version for almost 4 years and bought the iPhone and iPad versions the moment they were released. These 3 apps keep my life organised.
Every time I explain to someone what OmniFocus is and how I use it, they're almost always impressed but typically say it's too expensive. Yes, all versions of OmniFocus cost more than the usual task management app, but there's good reason for it. They're the best.
If you've been on the fence, both Omnifocus for Mac and OmniFocus for iPad are on sale at 50% off right now.