I've downloaded most of the free ones and even paid for one. The one I settled on is "tasks". It has definately been the simplest to use without signing up for some online service. No bells or whistles. Just todo list.
I tried many apps which seems to be good for some specific purposes.
Here I list out a comparison of apps that I'm still using on my iPhone
'Clear' is a different app which uses more gestures. I like the animation and sounds when I swipe the task as completed and adding new task to the list. But getting an overall picture of my to-dos is not possible.
'Orderly' is much simpler and has a great visual interface that gives a nice snapshot of my current to-dos. It reorders the to-dos for the follow up days based on the due date. It simply reflects what I think.