I used to use "The Hit List" for a long, long time before I moved to the (cross-platform) DoIt.im instead.
If you want a rock-solid, fast, GTD app that does all that you need for Mac and iOS, The Hit List is the one for you. Give it a try! (I have no affiliation with the company, just a very happy past user).
I am an OmniFocus user myself, but The Hit List does a lot of things well. Granted, I haven't used it in years but it support nested projects, tags and actions. A killer feature is its ability to blow up an action to full screen and show a large note area, and is the biggest thing I miss in OmniFocus.