I've looked at quite a few of the online systems and decided that Toodledo was the best. I know that some complain that the general look/feel is not as pretty as others (Nozbe, RTM, etc), it is very powerful and flexible so can be molded to pretty much anything you need and does GTD well. And it syncs with a few stable iPhone apps (Toodledo's own app, and my favourite Appigdo Todo).