Visio
Microsoft Visio is recognized for its robust set of features to help anyone create quality flowcharts and data-flow diagrams.
Pros
Visio provides drag-and-drop functionality and the ability to annotate diagrams. Both are helpful when creating a diagram for your system.
Cons
The application doesn't offer automated threat-modeling capabilities. As a result, there's no threat-generation engine to help engineers brainstorm threats and risk-reduction strategies.
Note
If you use Visio, you'll be responsible for threat modeling each element and interaction manually.