Quire based on a few key features:
1. Three-state task model 'to do' (open), 'in progress', and 'completed', with a long list of other metadata, like due dates, task assignment, comments, file attachments, etc.
2. Markdown support in task descriptions and comments
3. Organizations/Projects/Tasks -- invite different groups of people to organizations, share projects in organizations either publicly or by invitation, tasks are defined in projects
4. task lists -- tasks can be dragged, dropped, subtasked
5. Google calendar integration -- only one-way from quire to calendar
I hear they are working on Slack integration