I use Remember the Milk[1] and it has served me well for the past couple of years. I started off with the template presented in this post[2] from the RTM blog and it's a great basic implementation of GTD.

The advanced search functionality to combine items from multiple lists into a single view is my favorite feature. The keyboard shortcuts, quick entry and iOS/Android apps make it so that it's always easy to input something into my trusted system, whether I'm near a computer or on the go.

[1] https://www.rememberthemilk.com/ [2] http://blog.rememberthemilk.com/post/116665489183/guest-post...