+1 for Ultimate To Do List.
Actually, I've only used the Android phone version, but I've done so for a couple of years now. Prior to that, I tried a number of android phone apps to sync with ToodleDo and ran into a variety of syncing problems with many of them. Not so with this one - it's ROCK SOLID and provides full functionality. Some folks will complain of the "ugly" UI, and yes, it's not the prettiest I've ever seen, but the reliability and functionality way, way overshadow that for me.