Comment by karmelapple

Totally agreed - Things for Mac/iOS/iPad/Watch is a great ecosystem and Just Works™.

I started by reading the GTD book, and then tried lots of different apps, but Things for Mac by Cultured Code requires the least work and conforms roughly to the GTD approach. I don't use the strict GTD approach, but its approach to quickly writing down ideas and thoughts has shaped a lot of how I operate at work and even in my personal life.