I have used several to do apps for the iPhone, but have been using Toodledo for some time now, and I thought I would share my experience.
The best part about Toodledo is that it syncs to an online web app. Your to do list is safely backed up and available online. If you’re sitting at the computer and would like to add to or manage your lists, you can log on to the web app, and the next time you look at your list on your phone on your Toodledo app, your list is synced and everything is always up to date. Additionally, I have an iPad, and my list is available and up to date there too. This is great!
The app let’s you categorize items in folders, and sort them a number of ways, but typically I sort items by priority (I think there are five levels) first, then alphabetically.
I also “star” items to bring even more attention to certain items.
Great app, Toodledo