Is there a simple To Do app that works on iPhone, iPad, and Macbook Pro?
Most people are looking to choose between the hundreds of Apps that exist in this class.

For most, you can't beat the free Reminders app with iCloud syncing. 

It really boils down to what you call simple and how you work. Some find the simple design of Cheddar or Clear to be great while others dislike them strongly. You could easily spend more than $100 for a best of class solution like OmniFocus or Things

If the free reminders app (in conjunction with iCloud's notes and calendar integration), then you can have some more specific requirements from which to choose from the many, many other solutions from the App Store (as most of the apps in this class have adopted the Mac App store and iOS App Store as there is little downside for them to comply with the requirements of sandboxing and code signing).