Ask HN: What's the best SCRUM tool out there right now?

It seems that so so many tools can't properly handle a good work breakdown structure - you can break your tasks down, but some arbitrary level of work (usually first or second tier tasks) is the atomic unit of work for a sprint.

I'm looking for Kanban, sub projects, hierarchical tasks, control over estimating, and not much else.

The critical function that none of the software I'm looking at can really do is putting a single sub-task from a story into a sprint.