I really like "To Do" by Appigo.
There are separate versions for the iPhone and iPad, both are really nicely designed. And it'll sync with toodledo.com accounts, so accessing your lists, adding, editing, marking complete, etc., from the web is easy.
The app will do nested to-do items and also has categories like "Call" a person and "Email" a person and will integrate with your iOS devices' address book.
It's very much worth the couple of bucks each app costs. I've also paid for a "pro" account at toodledo.com (standard accounts are free), which allows for proper cloud syncing of nested and advanced items. Pro accounts are something like $15 for the year, which is very reasonable and worth it, IMHO.
You can also set up via toodledo.com a subscribed calendar in iCal format so you can see due dates and such in iCal on the Mac and on your iDevices' calendar app (also works with Google/Gmail calendar and Outlook on the PC).
Appigo, the maker of the app, also has its own cloud-based sync service but I think it's mainly to get data from one iOS device to another, without web-based access, though I don't know all the specifics (since I've been a toodledo user for so long).
