I am going to explain what Terran units are best for a situation:

Marine: All around great unit, they can take out Hydra's easily when backed up with medics and stim, same with Dragoons. Watch out for melee attackers like zerglings, zealots, dark templar, ultralisks, and firebats. Watch out especially for siege tanks, lurkers, and reavers. Marines are very good for taking out small aircraft. A Terran player should always have marines around their base. Make sure you use them in groups of at least 8, backed up with 2 medics.

Firebat: These guys take a more defined role, seeing as how their attacks and attack range is limited. Use them particularly against other melee attackers such as dark templar, zealots, and zerglings. The only melee attacker it will suck against would be the ultralisk. I've also actually seen firebats burn through lurkers pretty fast, but they have to get up in range to attack it first, which they die pretty easily. A very good unit for incenerating buildings, but not guard towers.

Ghost: Their attack may be limited, and they may have small health, but it is always smart to keep a lot of these guys around when fighting Terran and especially Protoss. You know why? LOCKDOWN. An EXTREMELY helpful technique. Lets say if you had 4 ghosts with 200 energy, and the enemy was attacking with 8 carriers, well, LOCK EM DOWN! They will be helpless. Just make sure you have other units around to kill the locked down unit. And also, can't forget to mention their nuking and cloaking ability!

Medics: Always keep medics around your infantry. And also keep at least one or two in your mineral field in case you get a suprise attack. They will keep your workers alive long enough for you to bring your troops over. On with their special abilities. Optic Flair. Protoss annoying you with observers? Render their usage helpless! Just flare it and it can't detect, or see worth crap! And restoration.. whenever you attack zerg, bring medics along no matter what unit your using against them! Because when a gay defiler comes and plagues your guys, just use restoration on your guys.

On to Factory made units:

Vulture: This unit is probably the best unit to scout the map early on. Not only does it make a good scouting unit, but it can also lay mines! Mines can be very helpful at times (if the enemy doesn't bring a detector when attacking). One game, I had mines all over, and it completely wipped out my opponents attack group! Also for Vultures attack ability, unfortunetly, is only good for taking out small infantry as: zealots, dark templar, marines, zerglings and firebats. Otherwise this unit does NOT make a good attacker.

Tanks: Great for defense, especially when in siege mode. Can also be very useful for offense, even better if you get them on higher ground. If you see high ground around an opponents base, just simply put tanks up their in siege mode and start blasting away. Make sure to always bring support for your tanks, because melee attackers can easily kill them. Tanks are made to take out these kind of units: dragoons, vultures, marines, firebats, goliaths, hydras, and reavers.

Goliath: These units can be pretty good at taking out ground base units (not as good as marines!), but they really shine for the use of anti air. With the charon booster upgrade they make something like a siege tank for air units (almost, but not quite). If you see that your opponent is going balistic with air units, get a ton of these guys. Air units to use goliaths against: Battlecruisers, Scouts, Wraiths, Corsairs, Valkyries, Carriers, mutalisks, arbiters, scourge, and devourers. Just watch out for guardians.

Now Air Units:

Wraith: A deciding factor in many of my wins, the Wraith is very helpful. But really only when cloaked! If you see that your opponent is trying to expand, take their expansion out before it is built. Make sure that you scan for observers when sending your Wraiths in to kill. Also, when the opponent is attacking your base, have your Wraiths creep up behind the attack group, take out the detector, and start blasting away at the attackers (Warning: Terrans have Comsat, so be careful when using Wraiths.) Remember: Wraiths can take out Air Units fast, but only when they aren't detected. Make sure you only use Wraiths when you can cloak, or take out units that can't attack air. Otherwise, if the opponent has detection, wraiths die rather quickly.

Dropship: Second fastest transport in the game.

Valkyrie: These units build rather fast, and have a unique attack pattern. It launches missles at the enemy, but the missles affect everyone in a 3x3 square (think that's accurate). So in other words, these vessels can take out grouped air units very quickly. But never use these to take out large aircraft such as Battlecruisers and Carriers. Otherwise they are good against: Scouts, Wraiths, Corsairs, Devourers (yes, I tested that out too), mutalisks, guardians, and overlords. Also, they are very fast, so you can hunt down enemy transports with them. Warning: Watch out for Scourge!!!

Battlecruiser: A very, very expensive unit. Although a powerful one at that. It takes really long to build, and will suck your recources dry, so make sure you have a lot of supply depots build, and you have a lot of recources to spare before ordering some up. Although zerg players may send scourge at these vessels, but BC's can usually destroy them before the scourge get to them. They are very good against most of the games units besides these one's: Wraiths (when cloaked) and Scouts. A group of battlecruisers can take on a group of carriers, but you have to use them right. Example: use yamato gun on the carriers, and then start attacking the carriers, make sure the intersepters don't distract them. Make sure you research and use yamato gun to take out guard towers.

There you go, don't be hard on me for typos.

CreepColony's Opinion: Firebats are NOT good for lurkers or any buildings. Vultures ARE good for attacking early in the game, especially against Protoss. BattleCruisers are best used as support units for tanks and stuff.