Unit Overview
Updated for HOTS!
Widow Mine
Widow mines are great, but largely situational units, and are usually too gimmicky to include as a large percent of your army. I love using them in pack of fours sitting under an armory or a banshee, and once the Mutas take the bait, KABOOM all dead. They are good to drop early game vs toss to punish any1G expand to stargate metagame builds they have vs mech.
Hellbat
These guys are what makes mech possible in HoTS despite all the anti Mech units introduced. They stand toe to toe with Zealots, Stalkers, Hydra by price (since they don't cost gas). Hellbat drops are your 1 way ticket to Diamond guranteed. Since the release most people's hellbat drop defense has fallen out of practice.
Medivac
In WOL, they were pretty situational and usually just for hellion drops. Now with boost, they are super effective and maneuvering your slow units around to defend your farther bases. Also, they can be used to drop Mines and Hellbats in addition to Hellions. On Yeonsu, the corner (destructable rocks) island bases has a ledge where you can drop high ground tanks and deny mining at your opponents 3rd, and if they have no air/drop capability you can score a couple of free unit kills while they're killing the rocks as well.
Thor
Playing Thor Banshee and Helion Tank Viking are vastly different, so if you play Thor based like you do Tank based, well you're doing it wrong :). If you need both Thors and Tanks (vs Mutas and Banshees for example) you should only get the bare minimum needed and favor tanks.
vT: Get them if you lose air superiority and don't want to catch up in vikings.
vZ: Only build these if they're going mutas. Make sure you don't overbuild so you're ready when he transitions into SH.
Banshee
With the shared upgrades buff, these guys are lean mean killing machines. Great in all matchups, good general purpose unit. Exceptional at sniping tanks in standoffs, baiting scans and in general forcing your opponent to micro more then they're used to :). For harass, it takes about 6 probe, 8 drone, or 7 scv kills to be cost efficient before it dies. If it survives, well any amount of damage is great :)
Hellion
This unit has utility all game. You should use them like the zerg uses mutas. Always having them on the map. This really trains your multitask and you'll how much better you will get in terms of offensive and defensive multitasking after juggling squads of these bad boys. You should have enough experience from Bio to understand these guys.
Tank
The powerhouse. At first weak, and dependent on building placement, chokes and helion support to be useful. Later on, as tank number grow past 18, becomes a force to be reckoned with. They are the most cost efficient when spaced about 2 units away from each other in a 2D lattice. If used defensively, you may want to space them out further to defend more space
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Viking
The Third most useful unit. These things stop you from losing to
1. Warp Prism harass
2. Zealot/Immortal bombs
3. Blink Stalkers(observer snipe)
4. Protoss countering your tech path
5. Protoss knowing when you expand way earlier than you should
6. Protoss sudden air switches
7. Collosus Sharking/Potshots
8. Final stand defenses versus mass Immortal onslaught before ghosts come save the day.
and the list goes on and on (and this is just for protoss :) )
Raven
The fourth most useful unit. To kill carriers, grab a clump of hellions, cast HSM on the ones in the back row, and move the clump directly under the carriers. This way, you won't lose ravens to feedback/carrier range and the protoss can't run back and waste your energy either.
Ghost
Great in Sky vs Sky late game TvT to EMP the other raven cloud. Always useful in protoss. If you get them early (~3 base) you have a lot of stopping power and space control, but you lose map control since you delay your banshees, ravens and their upgrades.
BattleCruiser
While ridiculous looking, building 1 or 2 when you are rich enough isn't that bad of an idea. The point is to force an over-reaction from the opponent, and enjoy how supply-efficient your army will be from these.
