Best task manager: OmniFocus
If you're a project manager who needs serious task management capabilities, such as timelines, multimedia annotations, multiple assignees, calendar integration, and hierarchical steps, OmniFocus is the gold standard for iOS and OS X. If all you need is a to-do list, the iPad's built-in Reminders app is primitive but serviceable.
Developer: Omni Group
Compatibility: iPad (iPhone version: $19.99)