Chris Bowler, writing for The Sweet Setup, picked Things 3 by Cultured Code (iPhone/Apple Watch | iPad | Mac) as the best GTD app. I have been using it since the beta and I couldn’t agree with their pick more. I would go as far as saying that is the best designed app that I use.
My only complaint is that they didn’t use version 3 (which was a paid upgrade) as an opportunity to combine the iPhone and iPad versions. But, while charging separate for the iPhone and the iPad seems a little off in 2018, I won’t complain too much about developers doing whatever they need to to stay afloat in the App Store. However, I would have probably preferred for them to go the same route as Ulysses and offer a subscription that enabled the app across all platforms.
If you want to learn more about how to squeeze as much power from Things as you can, Shawn Blanc is releasing a video course for Things on The Sweet Setup next month.