It should provide such feaures: - clean interface; - quick non-obstructive task adding; - task/tasklist delegation and collaboration; - subtasks would be cool; - comments with attachments; - due dates/deadlines; - priorities for tasks; - web and mobile (especially iOS) apps; - notifications (mail, push for mobile).

As far my favourite is getflow.com, but it's pricing sucks (price/per user is rubbish). Any suggestions?