Task Manager App

Wunderlist. I use it every day and it works great.

 

I recommend going with apps that integrate with iCloud Reminders. That way, if your need ever changes, you can switch to another app without losing data. Apple may also improve Reminders (on iOS) and iCal (on Mac).

While not perfect, my favorite on iOS is 2Do ($7 but frequently on sale at $3). You may need to play with the settings to your liking, but I like it much better than Reminders, Todo (from Appigo), Toodledo, Wunderlist.

On the Mac side, I like QuickCal. That said, Fantastical (which I prefer for calendar) will be adding tasks soon.