First of all, if you haven't played Starcraft before, do so now. Well, because I'm bored in study hall and really have nothing else to do, here is a list of the top 13 Starcraft units (In order).

13. Wraith- Not very strong by itself, but combined with an air force of Valks and Battlecruisers it becomes a powerful killing machine.
12. Firebat- Not much use against the Protoss, but when faced with a Zerg force it because the Terrans most valuable land unit.
11. Devourer- Not strong on it's own, but great at pissing the enemy off with its "powers".
10. Guardian- Dominates buildings. So much.
9. Mutalisk- Not great, but together with Guardians they make an impressive force.
8. Templar- I hate them. They can all die.
7. Zerglings- Zerg Rush, 'nuff said.
6. Marines- Versatile unit, handy to have around. Great for base defense when in bunkers, even better with a few medics.
5. Medics- Combined with a Marine/Firebat force, they become near unstoppable.
4. Hydralisks- Like a Marine, but stronger.
3. Battlecruisers- The pound air, and ground, and everything else.
2. Carriers- Slightly better then a Battlecruiser considering their shields recharge quite fast.
1. Scourge- Picture this: you just build an epic, fully upgraded group of 12 Battlecruisers for near 10,000 minerals. You prepare to move on the enemy Zerg when BAM! 24 Scourge come from no where and destroy all of your Battlecruisers. The worst part? They are only 50 minerals...For 2.