What would be an ideal to-do app to use in conjunction with Omnifocus...?

I'm a bit confused, why would you want another to-do app when Omnifocus is a to-do app?

But if you want something simple, have a look at TaskPaper.

 

OmniFocus 2 is a wonderful ToDo app in and of itself. I use OmniFocus 2 on my iPad, iPad mini , iPhone and Mac. I love the app and it is my main to do app.

That being said though, it was not my first task app on the iPhone. My first iPhone was the iPhone 3G back in 2008 and the first task app I got was ToDo from Appigo. I have had the app on my iPhone since, while OmniFocus has only been on the past 2-3 years. ToDo is a pretty decent app and both OmniFocus and ToDo work well for me and each has it's unique features that keeps me using both. However I do find myself relying more and more on OmniFocus 2.

 

I usually just use the stock reminders app (and view them in fantastical).