I've tried every GTD app out there. Some are very good and I've enjoyed using. I especially liked the apps that are cross platform - online, Windows, Android, etc. However, I always seem to eventually lose steam and constantly reboot my whole GTD system or move everything over to a new app. Recently, I realized two things - 1) GTD is a good idea in theory, but way too complex to keep up on and 2) as much as I hate to admit it, web apps aren't always the answer.
Let me expand on each. GTD is a great system - it's super organized and does what it's supposed to do - keeps you on task. It's just not practical - it's too hard to keep up with categories and contexts and projects and tags. It doesn't matter which app I use - it's just too complicated - it's too much effort - too much time. But I still like most of the core ideas in GTD.
I consider myself a computer guy and love the idea of keeping my systems online where I always have access ot them. I loved trying all of the different GTD apps - probably too much so. I was constantly trying different apps - moving tasks and projects out of one app into another. Speed and ease of use was always a factor. I'd heard people mention that pen and paper was the best approach, but I never took it seriously - I thought, "they must just have lists, they're not doing real GTD, they probably don't 'get it'". But after failing so many times at GTD, I needed a new approach - so I tried pen and paper - and I love it.
I have a composition notebook and a pen. It's mobile, it's easier to write on then a smart phone, I can bend it to work for me, the speed and ease of use is amazing, and it just feels good. I think the last thing is the most important thing - it just feels good to keep track of your tasks on pen and paper. Maybe it's the break away from the computer for a minute or maybe it's something else. But, it just feels good. So, at least for now anways, I've found the best GTD app of all time - pen and paper - simple.