Which 3D printer should I use to make custom miniatures for a tabletop game?
The Form 2 will definitely give the best results for your application.

Both the Fortus and the Dimension use FDM, which builds the model up using a bead of molten plastic. Because this bead is typically 0.5mm in diameter, this strongly limits the details you can print.

The Form 2 uses liquid resin, that is cured by scanning it with a laser. This process is called SLA. The laser produces a 0.14mm dot, and so you can produce much finer details. The Form 2 can also use much thinner layers (down to 0.025mm v.s. the 0.178mm of the Fortus) so the models will be much smoother.

To make small miniatures SLA is much more suitable than FDM.