I have an account on RTM, but I don't use it. It's interesting, but it pigeonholes you into its own way of thinking about GTD. If you don't want to do it exactly their way, you're kind of out of luck.

I think I've tried Toodledo in the past, and I've looked at Nozbe, but both seemed to suffer from similar ailments. Neither is very customizable, but if you want to operate the way any one of them is set up, you'll probably be fine. For a while I use Hiveminder, which is another "full-service" GTD webapp that fit my methods better.

However, these days I use SimpleGTD. I know I harp on that service all over these forums, but it really does do GTD in a very basic and flexible way. It leaves room for you to deal with the data however you want, and while it doesn't provide any built-in coolness it allows you to interact with the raw information pretty easily. My favorite part is that it can keep track of master Action lists but only display the subset designated as Next Actions in your context lists.