For example, A hydralisk.

Mine is the incediary pigs from Rome: Total War.

Protoss Archon - I love the sound of their attacks, and seeing a group of 10 of them taking on a group of 20 or so wraiths or mutalisks. Regenerating shields is a plus too.

"POWER OVERWHELMING"

For me it's currently using swarms of Ohm bots from Universe at War.

Flowing a massive wave of them into an enemy staging area results in some very satisfying chaos, and the added bonus of having the occasional one leap out of the horde onto an enemy and overload itself to do lots of extra damage is just icing on the cake.

MonkeyLord from Supreme Commander.

Just a great, well designed unit. Very threatening in appearance, very fast compared to it's counterparts, and sported a variety of weaponry (most notably it's giant microwave beam of death). What was probably most scary about it is that unlike many of the Experimentals in the game, the MonkeyLord was very reasonably priced and it's build time allowed them to come out fairly early in a match if a Cybran player decided to build one (although the less cool looking Soul Ripper became the Experimental unit that most players raced for). But seeing a few MonkeyLords walking across a battle sweeping over enemies with it's laser backed up by an army of Loyalists is probably the one of the most beautiful things I've seen in a game.

After that would probably be the Taiidani Heavy Cruiser. A giant unit, with basically no anti-fighter weaponry but that was besides the point, since it's main guns fired rounds that were almost the size of a regular fighter craft. Seeing one of these monsters when all I had were Destroyers was never a fun feeling.

I do like subtle stealthy units and all, but some of the giant heavy hitters can be great fun in RTS games.

Dreadnought from Dawn of War. Just watching it pick up Orks and smash em back down again in to a bloody pulp... awesomeness.

X Blink Singularity Deathsphere from Sid Meier's Alpha Centauri. (Flying fortress that ignores walls and shields,fires a Singularity gun and uses nerve gas)

Second would probably be the Planet Busters from the same game, especially with the top-tier reactor. There's something empowering about having the armament to rip a small continent to shreds.

Tesla tanks from the expansion for C&C red alert purely for the charge up noise.
*SHWOOOOOOO... KZKKZKZKZKZZZ*

Peons, warcraftIII..

"work work"

Tesla Coil, Red Alert.

Pathfinders from Command and Conquer Generals. Three of those guys could hold off a swarm of Chinese soldiers.

Oohh...
Crazy Ivan, Red Alert 2. "I lost a bomb, do you...have eet?"
That you can combine him with Yuri to send hordes of cows, civilian cars, etc. into the enemy base and harass them doesn't hurt either!

The Overlord Tank, Command and Conquer Generals. "They all look PUNY from here!".
...yeah. Versatile, feared, powerful, can run over other tanks and house five rocket launcher units. What's not to love?

Dreadnaught, Dawn of War: Least balanced unit in the history of unbalanced units. THREE of these things is all I ever needed to complete any given mission in the original game (including the last one), no matter what. Later on they became a bit more balanced, but still fearsome powerhouses. The name fits very well.

Spiderbombs from Warlord's Battlecry 2.

With a fully decked out merchant hero, they cost 12 metal and 12 crystal each. In comparison, every mine gives you SEVERAL metal and crystal per SECOND and there are quite a few mines on most maps. Fully upgraded, the things are capable of leveling towers (the most stalwart and powerful of defenses in the game) with just 48 crystal and metal (theoretically, in reality it costs more, and in reality regular golems do it with way more cost effectiveness but are way slower to do so), making them the most effective base-buster I have ever seen in a game. Yes, the game is balanced.

Basically, imagine a zergling rush that works mid-game, with explosions. Late game, you can have a literal INFINITE stream of them rushing in. Especially fun to just put rally points in the enemy base.

Tiny explosions, all over the map. Brings a tear to my eye.

Gotland Swordsman and Longbowmen from mevieval Total 2 war

The Marine Frigate in Homeworld 2:
Fast, manouverable, fairly well armoured.
It goes over to enemy Capital ships like Dreadnoughts and seizes control of them, making them your units to command. :D!!!

The Scout from homeworld, fast, cheap and great for taking out a resource operation in a massive 100 unit kamikaze attack. Also have to include the GLA worker in C&C Generals. "Can I have some shoes???"

Design- The FatBoy , Supreme Commander + The Monkey Lord, Supreme Commander

Style- The Tesla Coils, RA2 + The Desolator RA2 (" There Goes The Neighborhood!")

Strategy -Obsidian Statues/Meatwagons/Necromancers In WC3 + Prism Tanks, Mirage Tanks RA2 + Carryalls From TibSun And Dune

Structure
a)Style - Nuke Silo RA2 + Ziggurats, Arcane Sanctums, Boneyards, Adn Necropolises from WC3
B) Concept - Cloning Facilities RA2, SupCom Heavy Artillery
P.S. I Luved Age Of Mythology's Wonders.

Kill ratios- Apocalypse Tanks and Prism Tanks, RA2

CAn't think!!!

They were hard to use in a serious game, but I frickin loved the prism tanks from Red Alert 2. If you got 4-5 of those into the enemy base when they weren't around to defend it you could ruin their shit so fast it wasn't even funny.

Definetly the Fatboy from Sup Com, best defense you could ever have. Put up a line of those in front of your base and no ground attack can get in, combine it with some flak and sam towers and you can waste everything. They've got 4 3-barrel artillery cannons, some MGs and light anti-air. The overlapping shields rock aswell, not to mention the factory (that I never used, I always just rolled a few of em to the edge of the enemies base and laid waste to it from there)

I'm a big fan of Infantry in my games, so my call definently goes out to the Paratroopers in Company of Heroes. Micromanaged properly and kept updated those guys can wreck anything, they're just so versatile as well as able to reinforce anywhere on the map.

That said, as far as infantry go the Space Marines basic infantry, yeah, they got style! PURGE, CLEANSE, KILL! Also loving the Dismounted Knights in Medieval 2: Total War. Nothing says power like fielding an army of glimmering fully armored infantry that have the damage to cause some serious casualties and the armor to take a halluva beating before going down. Expensive as hell to maintain though, that's why you don't see many forces like that!

Inflitrators from Star Wars: Empire at war

or

Lurkers from Starcraft...lurker drop ENDS you base, lol

I do love the homeworld posts i read as well, those are hilarious ideas for scout fighters!

two words "Power Overwhelming"

and you cant help but love those Zerglings. they rush you beat the hell outa' you and, GOD I HATE THEM!

"Broadside Battlesuit redeploying" I forgot about the almighty unstoppable KROOTOX

British PIAT Commandos and tetrarchs close runners up.

For a combination of kick-ass firepower, attitude and iconic representation, I gotta vote the Mammoth Mk IIIs from C&C3.

"Drop 'em!"

Dreadnaught for DoW, and the chaplain from DoW: Winter Assualt.

mammoth from c&c

The Tesla Trooper!

"Extra Crispy!"

Lovethor:
Tau stealth troopers from Dawn of War- Dark crusade
Watching them as they fire upon infantry or tanks in stealth is just great

Harrasing noobs with those is just too funny.

The blue-painted Gallic swordsmen from _Walls of Rome_. They were *awesome* to use to counter-attack any footholds the enemy had gotten up on your walls.

It's probably not my Absolute Favourite, but I'm fond of an Ultramarine with a powerfist in WH40K: Chaos Gate. They were so bloody visceral.

Yuri from RA2
Look deeply into my eyes

Crazy Ivan from RA2
I lost a bomb, do you have it?

Tanya from Red Alert
hahaha

CHOPPAS!! from Warhammer 40k.

IT NOW BELONGS TO DA ORCS!!!

Grey Knights from Dark Crusade. "This is the judgement of the righteous! Scum!"

For me? it's gotta be the Krogoth from Total Annihilation, that thing needed it's own structure to be built!

second for me were the Beast Cruise Missiles from Homeworld: Cataclysm, Kamikazi-ing them into your opponents attack and then watching their own forces kill each other was great, awesome for ambushes too.

God...there are so many...

The Wizards or whatever the name was in Warcraft (I think 1 but it may have been 2 or even both) because of polymorph. Nothing was quite as satisfying as turning your enemy into sheep.

I loved every unit in Warhammer but I think my absolute favorite were the nob squads. A fully reinforced nob squad was a wonder to watch go to work. They were just a wall of green meat that hit anything hard and the sound of their claws going to work was great. A close second from Warhammer would be the Chaos walking tank doohickey just because it looked cool. It was fairly useless as artillery and took too much damage for late game tanking but they are just great.

Company of Heroes also sports a ton of great units. All the tanks have a very satisfying attack and I loved the infantry because you had to love them or you would lose. Above all I have to say a ranked up truck with those huge anti-air guns is my favorite. Watching that thing go to work on a field of infantry is and always will be a joy.

Honorable mention to the Witch Doctor in WC3, Aurora Bomber in CnC:Generals, APC+any infantry in CnC3, Uber spearman in Civ3 who could kill a tank, and Trebuchet in Age of Empires II (which is a difficult decision because the elephants were made of so much win).

EDIT: I can't even choose a unit in any of the Homeworlds, they were all so great. I suppose nothing will ever compare to hyperspacing a battlecruiser, frigates and a carrier filled to the brim with fighters right next to an enemy base but that's a bunch of units and not just one.

Tanks and pikemen from Rise of Nations.