When it comes to GTD, after a year and a half of finding the ultimate solution on Android/Windows, I have settled for Nirvana HQ. Nirvana meets the following criteria which in my mind, are critical in order for a solution to qualify as a GTD solution.
Firstly, it achieves that Holy Grail facility of "auto next action". When you create multiple projects (I believe you need to be a subscribing customer in order to be able to create more than five projects) the various "next" views that you utilise (by context, area etc) will only be populated by the top tasks in your various projects. When you mark a tasks completed in that view, Nirvana will then automatically refresh your current "next" task list by selecting the next sequential task from the project in question.
Nirvana HQ is platform agnostic. The main interface is web based. The features are amazing in such a cheap solution. (Free or $39US per year). They have a mobile site for use on tablets and mobile phones.
There is offline access too. Currently there is an iPhone app (which you can also use on the iPad) and the Android client is in beta. I have been part of the alpha and now beta program.
They allow tagging tasks with multiple contexts. Some GTD solutions only allow you to tag a task with one context. This is a good discipline but sometimes it can be a little inflexible.
In addition, the developers and community are incredibly helpful and active.
If you are looking for the ultimate solution, check them out. Seriously, after one and a half years, I don't think it gets any better than this.
If you reckon there is something better, I'd love to hear from you!