To start off, I'm a Zerg player for the most part, so I'm obviously not going to be so well-versed in Terran as I am in Zerg, or even Protoss, but I'll try my best to not be a jerk about it. And when I say "I'll try my best" I mean, I really won't, and no one reads this anyway. But that's ok, I prefer lurking. (Yes, that was a pun. Because I like Zerg...Lurkers...stfu)
Terran:
-Goliath: 
They're just practical. Strap a Missile Turret to a roided-out mecha-marine's back, and you've got yourself a Goliath. They also just sound bad-ass. "Goliath online." "Go ahead TACCOM." It's just too cool.
-Medic:
Obviously when I play Terran, like anyone who ever hopes to win, I use loads of Marines and Firebats. LOADS OF THEM. So to annoy my opponents as much as possible, and to get as much use out of these forces as I can, I stick a few Medics in the mix. When I first started playing, years and years ago, I seriously underestimated the power of the Medic, and it cost me dearly when groups of ten or twelve Marines were able to overtake 20 or so Hydralisks if they had a couple medics in there. I don't frequently use Medic abilities unless a Zerg player has plagued one of my upper-register units, then I restore it. But I really don't use Optical Flare much at all.
-Ghost: 
Three words; Cloak, Lockdown, Nuke. The Ghost isn't one of the most versatile units in the game. It pretty much just acts as a ninja. However, it's fairly useful, especially when that Shuttle plops a Reaver right behind your mineral field... right next to your 30 SCVs...and you forgot to back them up. "HOLY HELL, LOCK THAT SHIT DOWN! LOCK IT DOWN!" *boom, boom- vssshewww*
-Science Vessel:
I get the feeling that I'm one of the few people who actually enjoy using specialty units like the Vessel, the Defiler, the Queen, etc. Obviously I like EMP Shockwave for Archons, because they're scary as all bleeding hell, and it gets rid of those pretty darn fast. And Irradiate is also an obvious choice for anti-Zerg battles.
Protoss:
Photon Cannon:
OK,OK, I know it's not a friggin' unit, but just hear me out. Back when I was just getting into StarCraft, in about...2000 or so, I picked up a strategy from a buddy of mine, who played Toss quite a bit. He essentially built a few zealots to defend his base sufficiently, then researched like hell, and warped in a ring of photon cannons all around his base. I'm not talking just a tiny ring, I'm talking about around 4 - 6 cannons deep, all the way around. The only thing you have to fear is Siege Tanks at long distance. It makes for a very interesting game, and usually players won't expect something this rediculous.
Dark Templar: 
Something that's permanently cloaked, and attacks with a big blade of energy, and just happens to be one of the most bad-ass units in the game is certainly one of my favorite. Fantastic for terrorizing small groups of marines, and even worse against Zerg, because they don't have a ComSat station.
Dragoon:
It's a walking Photon Cannon. What's not to love?
Reaver: 
A little impractical, but it gets the damage out there when you need it. Certainly something to be microed along with a Shuttle, so that it can move a little faster than 2 inches/hour.
ZERG:
-Hydralisk:
It has a second lower jaw that separates in the middle, and it shoots spines. Out of its face. OUT OF ITS FACE! ARARAGAAHHAHRAHRHAHHRA!!!
-Scourge:
I think that most Zerg players share the absolute delight that I experience when I see an "unprotected" Battlecruiser or Carrier just thinking it's all invincible and stuff. I think that one of the major flaws in the other two races is the belief that pure might will take the place of skillfully surrounding a unit, and biting its head off. There are some Terran and Protoss players who believe that their Carrier or Battlecruiser can fend for itself for thirty seconds while they switch out fresh troops.... Hah... ahaha... HAHAHAHAHAHHAAHAHHAhahahAHAHAHAHHAHAHGHAHGHA! Oh that's funny shit.
-Lurker:
I don't even know how Blizzard came up with the idea for the Lurker's attack, but they couldn't have made it any more cool. A row of spikes, from underground, that follows through, and damages everything in its path. Way too cool. And the fact that you can move this thing around, and burrow it at will, it's just sick. It looks really cool too.
-Defiler:
Towards the end of a game, the only real applications of the Zergling are scouting, lurking, tormenting undefended SCVs, and FEEDING THEM TO DEFILERS. The idea of Consume was the best thing since sliced Firebat. Now I can run a few Ultralisks over to one side of a base, and while all the defenses go there, I can plague the rest of it, and then bring in a couple cheap-o Mutalisks and wipe everything clean. Dark Swarm is also pretty darn useful once you get the hang of it. If you're a Zerg player, and you've ever had about a million of your units popped at once by Siege Tank fire, that you couldn't even see coming, Dark Swarm is the preventative. Swarm right over your units, (Splash will still occur, but damage won't in the full effect) and relocate your forces. While the tanks reposition, consume a Zergling or two, and Plague them. Now they're fucked. Serves em' right for being a SIEGE TANK WHORE.
-Queen:
Parasite is fantastic. It's damn near impossible to notice if the unit is going to stay near the opponent's base. Great for monitoring, and it lasts for a long, LONG time. Spawning Broodlings is an easy way to get rid of Reavers or Tanks, and the occasional Goliath that feels the need to hunt down your queen after placing a Parasite. Truth be told, I don't use Ensnare that much. As if that Reaver needed to be any slower.
I'm very excited for all of the new units/ upgrades, etc. that are coming in SC2, especially Zerg stuff. Spine crawlers, Nydus Worms, Banelings, Roaches, etc. Very very excited.