I've finally settled on Zoho Planner as my To Do list of choice. The more I use Zoho's suite of products the more I come to appreciate it.
The planner doesn't support task hierarchies, but I'm finding I don't really need that. In fact I find it helpful to streamline one's thinking into single tasks, not large tasks with sub-tasks.
Of course you do want some degree of categorization. Zoho Planner lets you make multiple lists, that display very nicely on the page. They can represent different projects. I'm finding this is sufficient for me as far as task decomposition is concerned.