They would be all totally baffled by the keyboard. Forest would just shoot the screen.
Seriously, there is no telling how they personally would do since most were leaders of men. In a simulation game Halleck might end of the best. However if you think in terms of eachs approach to war being instilled into a good gamer there would be quite a differences in their styles of play.
When I play the Union side in scenarios like Gettysburg I tend to use more of a Grant approach. I know I have the men and arms to grind the Rebels down so I do. No slippery tactics needed.
When playing the Rebels you have to make up for their smaller numbers and take advantage of the morale advantage. A Jackson/Lee style works best.
A Thomas style works in games like Chickamauga for the Union side but would be a loser in the reverse. Only a few scenarios reward a defensive approach except for one side like Antietam for the Rebels.
Forest was a raider so its hard to say how his style would work on Army level games.
One thing that is interesting to watch is how people's play style changes when they take on a blind scenario like those in Fight the War. Once you take away that 200 foot General knowledge it is surprising how some change from aggressive generals to passive or even hide in the bushes generals.