EDITED: I wanted to add a comment about the syncing problems others have mentioned - that problem ONLY applies to ToDo users who were using iCloud to sync with BEFORE iCloud changed to iCloud Drive - the change to iCloud Drive that Apple made to iCloud users is the root of the problem AND the developer has clear instructions on their website about how to fix this - I know, I was an existing ToDo user hit with the problem, but I followed the instructions and now my iCloud (Drive) is syncing perfectly. If you have never used ToDO before, you will not have this problem.
After recently updating both my OS (to El Capitan) and all of my versions of ToDo (Mac, iPhone, and iPad) to the most current, I now have everything syncing fine between all three via iCloud, although you can also sync using Dropbox or Appigo’s Todo Cloud.
I’ve tried a large number of ‘to do’ apps but overall this (Appigo ToDo) is my favorite for it’s simplicity (for just keeping track of simple ‘to do’s’ it’s pretty easy to start using with little need for instruction or tutorials) and because the interface is very well organized. I have my tasks categorized by Work, Projects (volunteer stuff), Family, and Personal. Start dates, due dates, and reminders are a snap to setup for those tasks that have to be done on or by a certain date, and changing priorities on the fly is simple also. I add things I have to do as they come up, and after adding notes and any subtasks, and assigning dates, I can then forget about it until they come up on my Focus list or I’m reminded to start them.
For more complex jobs, it also has the ability to turn tasks into ‘projects’, complete with sub-tasks and delegation to others and handle milestone dates.
One thing I would love to see the developers add is some options to customize the appearance (colors) - I love the App but I find the current color theme a bit hard on my eyes.