Terran
For Terran a good unit to mass is either wraiths and siege tanks. Siege tanks are just that, they are tanks that have two forms. Siege tanks can be in a mobile form where their attack is good with medium range. They can also go into a siege mode, where they are changed into stationary tanks that have a long-range cannon, which does immense damage. Their only drawback is that their attack rate is quite slow, they can't attack air units and they have a minimum range. If anyone is inside the minimum range or flying, they can’t be attacked. If you can manage two groups of siege tanks to "leapfrog" their way into someone’s base by having one group stay in siege mode while the other group approaches while covered by the stationary group, you can often deal great damage to the enemy. Just remember that enemies WILL counter siege tanks with overwhelming force or air units. Wraiths are flying ships that can use cloaking devices to become invisible. They benefit from being flying, ranged and invisible, thus they are very versatile. Their ground attack is quite crappy, but their air attack is pretty good. All your wraiths should be cloaked in combat so they cannot be seen. The only way for an enemy to see your cloaked units is with a detector. Each team has very few means of detecting and they are all counter able. With your wraiths your most important goal is to destroy their detectors because if you can't be seen, you can’t be attacked.
