I think a good question to start with is, what are you trying to get from your to-do app? Is it a simple to-do/checklist? Do you need multiple lists? Reminders? Do you need project management? Know what you need will help others be able to point you in the right direction.
There are plenty of To-Do apps that are compatible with both iOS and MacOS. Some of the most recommended ones are:
* Things 3
* Wunderlist
* Todoist
* 2Do
* Omnifocus 2
Personally, I use Omnifocus 2, but it's not quite easy to use - or cheap ($40 on iOS, $80 on MacOS, if I believe). It's more of a project manager than a simple to-do list, but it's incredibly helpful for my work situation (12 clients I see in different locations, 4 supervisees, additional administrative work).