For the Marine story you would want to go for the fastest troops which are the Synthetic who have a speed of 4. Once upgraded the speed increases by 1 making its speed: 5

For the Alien story you would want to go for the fastest troops which are the Runners who have a speed of 8. Once upgraded they still remain at a 8 speed.

For the Predator story you would want to go for the fastest troops which are the Brawlers and the Spear Masters who have a speed of 6. Once upgraded they still remain at a 6 speed.