2. Marine- Cost: 50 minerals 0 gas, 1 supply
One of the best units in the game, marines can be mass produced, take only one supply up, fire rapidly, can StimPack, attack air units and can get upgrades to their guns and armor. They can fire from bunkers, ect. For 50 minerals, it's quite a bargain.

3. Firebat- Cost: 50 minerals 25 gas, 1 supply
Another good unit, the fire bat can do must of the things a marine can. They are better attackers, but require to be at point blank to fire. The can StimPack, and I advise doing so, especially in the company of medics, because they move and fire twice as fast that way, despite the loss of HP.

4. Medic- (Brood War only) Cost: 50 minerals 25 gas, 1 supply
A very wise investment if you can StimPack. The medic cannot attack but can heal all conditions, including HP damage, and can also blind units, reducing their sight range to 1 matrix (Right in front of them). You'll want to put them in the middle of marines, firebats, SCV's and bunkers because if a bunker is destroyed, the marines can get more cover there.

5. Ghost- Cost: 25 minerals 75 gas, 1 supply
Ghosts can cloak, which makes them worthwhile, but in addition to that, they can launch nuclear missiles and cast "lockdown" on opponents. Lock down will do that- make all mechanical units it hits still for nearly 3 minutes (I think) They do 10 damage a hit, which is pretty good considering all their previous abilities.

6. Vulture- Cost: 75 minerals 0 gas, 2 supplies
Vultures make some of the best scouts in the game (After upgrades). Without Ion boosters, they move rather slowly. Otherwise, the can make Spider Mines, which do 125 damage, with splash damage affecting nearby units. They won't set off enemy spider mines, which is good.
            6a. Spider Mine- The Spider Mine is good for guarding resource areas, as you can see around where they are planted. The enemy can't see them, either. If you have bunkers, missile turrets, siege tanks, and spider mines, you probably won't be overtaken as long as you have adequate air protection.

7.Siege Tank- Cost: 150 minerals 100 gas, 2 supplies
Seige Tanks are some of the most effiecient killing machines around. In Tank Mode, they do 30 damage, but the real killer is the 70 damage they do in Siege mode. Despite the drawback of splash damage, if you can kill your assailant before they get to you, you won't have to worry about that. If you play Protoss or Zerg, the best way to kill these things is to use Zealots or Zerglings. If your opponent has 3 attacking you, spread out, the other two will fire on you and do splash damage to the friendly tank. Firebats will work, but they don't do full damage.

8.Goliath- Cost: 100 minerals 50 gas, 2 supplies
The Goliath is literally a walking missile turret, making them invaluable in defence. In Brood War, you can upgrade their range, hich helps immencely in island games, because they only cost 1/2 of a missile turret. They can fire on ground units, but i's not as good as a siege tank.

9. Wraith- Cost: 150 minerals 100 gas, 2 supplies
The Wraith is the basic flying unit for the Terrans, and it's good at that. The wraith can fire lasers at ground targets, but it's real ability are it's missiles. They are fired against air units, and they do 22 damage. They are fired rapidly, so you can get 44 damage in a really shot amount of time. All this is good, but the wraith can also cloak, which makes it better than scouts or mutilisks.

10. Dropship- Cost: 100 minerals 100 gas, 2 supplies
The dropship is used to ferry units across impassable terrain, so it's invaluable for raids on the backs of bases. They make OK island scouts. They're the second fastest shuttle.

11. Science Vessel- Cost: 100 minerals 225 gas, 2 supplies
The science vessel is one of those units with out attacks but with lots of abilities, which make it ok. It's abilities are defencive matrix, irradiate, and EMP shockwave. Defencive Matrix puts up a 250 HP shield around  the unit, which can make it good for small hit-and-run type missions. Irradiate puts a cloud around one unit, doing damage to it and the units around it. EMP shockwave sends a missile to one area, and will take out all shields and energy of the units in the blast.

12. Valkyrie- (Brood War only)Cost: 250 minerals 125 gas, 3 supplies
The Valkyrie is a unit that can only fire on other air units, but it fires volleys of missiles doing 5 damage each. That may not sound like much, but it fires 2 at a time, so thats 10 damage each hit, which will amount to something in no time.

13.Battlecruiser- Cost: 400 minerals 300 gas, 8 supplies