I think your choices depend on your strategy type and how much you grind your characters.
I enjoy having seperate squads that attack from different angles. Usually, Darkling, Shadowboxer, and Festus tackle enemies in the back corners, with Festus either healing or using mind blast
Then, I have Gamblin' Jack, Helena, and Main charge the front group
Finally, Grotius and Dorgon are used as a group, because Dorgons range and Grotius' power and mobility mean that they can kill quickly and efficiantly.
This strategy only works due to grinding. Personally, I trained Shadowboxer to the point where he was tied with my main, because he was faster, flying, had more health, and less damage. Also, Darkling has enough health that he doesn't need feedback, as long as festus is helping him. Next, I trained Festus to have the same Psy power as Darkling, making him a good assassin as well as a healer.