I just purchased Appigo's ToDo app for iOS and OS-X. Deciding factors for me were:
- GTD is supported
- iPad, iPhone and iMac versions available (iOS universal)
- sync using iCloud (also other services supported such as DropBox and Toodledo)
- very reasonable prices
Thus far, these apps look quite good, though the iMac version seems a bit older than the iOS version. This weekend I worked out a setup to manage all my tasks in a GTD way. I can also distinguish between work and private.
For me, this app strikes a nice balance between functionality and complexity. But like I said I only have it for a few days so time will tell how things work out.