I've tried many, many task management apps. For some reason or another I don't stick with them because they're either too complicated, too simple or too ugly.
I recently found Orchestra and so far I think it's the best I've used. It just seems to work how my brain works when I think about tasks.
For one thing, it looks quite beautiful. That's important to me.
You can create lists and order tasks by priority or due date. Sounds pretty standard but the method of switching sorting options is the most innovative and intuitive I've used.
There is a bar at the top of the app and you can toggle between starred (important) items, all items and all completed items. There is also a toggle next to it for lists - all lists or a selected lists.
This simple but powerful method of sorting is exactly what I didn't know I was looking for.
It even has robust task sharing functionality which I don't even use.
My biggest gripe with the app is that there's no iPad version (yet). I'm a completionist and it's annoying me that I don't have an iPad equivalent. Though there is a good web app it syncs with, which is very important to me.
I suggest you check it out if you're looking for something more sophisticated than Clear but not as involved as Omnifocus or Things.