MeisterTask's Slack integration is bi-directional: the inbound integration sends messages to a Slack channel whenever a task is created or dropped in a section (lane) in MeisterTask, whereas the outbound integration lets you insert tasks into a MeisterTask project directly from a Slack channel.
Asana has fantastic mechanisms for dealing with team collaboration. Not only can tasks be assigned to team members, but Asana lets follow tasks and even use hyperlinks to refer to team members within a task.
Trello works best with medium to small projects and with very high-level overviews. It is less effective for projects that require very granular management due to the fact that it becomes considerably more difficult to keep track of various cards and priorities as they are pushed off the screen.