I will start by looking at workflowy since you like its ability to focus on a single task when that's all you care about at that time. Thanks. (edit: I think workflowy may be the one I heard about here a while back, the name sounds somewhat familiar.)
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I've tried about a million, and have found Todoist.com and Workflowy.com to be the most aligned with how I manage to-dos.
They both allow hierarchal task lists like you want, and both have extremely clean interfaces.
Workflowy is particularly useful for focus, as it allows you to sort of "zoom in" on one task so you see only that task and its subtasks.
I will start by looking at workflowy since you like its ability to focus on a single task when that's all you care about at that time. Thanks. (edit: I think workflowy may be the one I heard about here a while back, the name sounds somewhat familiar.)