Outbreak is, for all intents and purposes, a zombie defense level... except you get flamethrowers and the chance to attack them without retaliation. If you wait to get the siege tanks first, at least 4 at each entrance, (with Psi Disruptors on each side, if you're that far), you literally cannot lose, even on brutal mode.
Gabriel Tosh throughout the entire set of Covert missions, especially in Breakout. He goes so far as to help you out throughout the previous two missions by exposing where all the Terrazine and minerals are, giving away a relative chunk of the map. No one else has been able to do that.
After the mission "Breakout":
Raynor:No one's been able to leave New Folsom for 50 years, and we broke it in a single afternoon.
Zeratul killing an Hydralisk with its own severed arm, and doing a Diagonal Cut on Kerrigan. (Though not anything vital)
The final Zeratul mission, In Utter Darkness, is basically this for the entire Protoss race, even if winning is completely impossible. Artanis's Rousing Speech when he arrives in his mothership is possibly the greatest part; it echoes Tassadar's from the end of the first game.
The Odin, a.k.a. the Big Dawg. It's an even bigger Thor prototype with massive upgrades and ridiculous firepower. To say that Tychus loves it is an understatement.
Tychus:The Big Dawg is all mine.
In the mission "Engine of Destruction", he takes control of it, leading to an escort mission that in itself it a CMOA for him.
"Media Blitz" is one giant CMOA for Raynor's Raiders, especially Tychus. The Raiders successfully hide the Odin's hi-jacking from the Dominion and call in a lot of favors to get it on the surface, in the center of the capital, with Tychus in the cockpit. He then turns on the Dominion, using the confusion to attack their bases and cripple them before they can figure out what's going on. They then hi-jack three transmission towers, fending off the renowned General Warfield's counter-attacks to hold them, and upload evidence that Mengsk used Psi Emitters to make the Zerg destroy Tarsonis and broadcast it to the entire Dominion. Cue revolt beginning, the media being turned against Mengsk to ruin his image, and a mild Villainous Breakdown when he begins to realize it as the reporters grill him. And that's just the awesomeness presented from a story standpoint—in-game you get to command the most powerful unit in the entire game (including the super-strong enemy heroes) and have three minutes to go nuts on three enemy bases. Just walking the thing around the streets causing cars to blow up when they crash into you and streetlights and other doodads collapse as you stomp past them, in addition to the chaos you're wreaking on your enemies. They might as well have called it Stuff Blowing Up: The Level. Let the intro tell you exactly what you're getting into.
Horner: Our raid on the Valhalla facility hasn't even been reported yet. The Dominion have no idea we stole the Odin. I had to call in every favor I had, but I got the Odin to the surface with Tychus inside. Raynor: Matt, you're a miracle worker. Must've been a complicated operation. Horner: You don't even want to know, sir. I'm not going to let anything keep us from exposing Mengsk. Raynor: So they're still expecting to film it at the UNN studios later today? Horner: That's right - and we won't disappoint them. Seeing as he's the only one with experience, Tychus will pilot the Odin and hit the studio guards with a surprise attack. They'll be confused for several minutes... and won't even know who to attack at first. Raynor: And during the confusion, we establish a forward base using this Command Center we "acquired" earlier. Pretty devious, Matt. I like it. Horner: If we can inflict enough damage during the sneak attack, making the broadcast should be relatively easy. Keep in mind, we'll only have a few minutes before the Dominion realize Tychus is in the Odin. Once they figure it out, they'll use everything they've got to stop us. Raynor: Whatever happens, we'll be giving Mengsk one hell of a broadcast. Let's do this.
And, in the aftermath, when Kate Lockwell tricks Mengsk into contradicting himself by broadcasting his Evil Gloating from the first game during a live press conference. The combination of that and previously released proof that Mengsk used psi emitters to lure the Zerg during the Fall of Tarsonis ignites a civil war against Mengsk, who suffers a Villainous Breakdown in the following cinematic.
The Dig. A bit of a Breather Level, it's designed to make sure you know how to shift-queue commands. If you don't, the level is impossible. If you do, it's an absolute joy. There's nothing quite as satisfying as disintegrating high-end Protoss units with a Frickin Laser Drill and watching the rest of the army splash harmlessly against your Siege Tank line. (And Bunkers. And Perdition Turrets. And...)
You can actually destroying the bases with a mass of units and using the laser to destroy units and pylons to limit casualties and de-power their structures as well as supply blocking them. Successively destroying the bases is an Instant-Win Condition and so very satisfying if you can pull it off.
The flashback cinematic of Tarsonis, where a pre-Infested Kerrigan takes down a Hydralisk with her rifle's underslung shotgun, followed by her resigned acceptance of her incoming fate, being surrounded by hundreds of Zerg while shooting a Death Glare at Mengsk's ship up in high orbit.
How about when Jimmy kicks Tychus' ass in the bar, despite the fact that the latter is clad in a suit of Power Armor, and even without it is about half again Jimmy's size? He then turns around to set his more doubting crewmen straight on just what's at stake.
If you talk to him later, he says that he got a poisoned dart in the arm, so he told the medics to cut off his arm so he could get back in the fight. Apparently, refusing to do that makes you a "pansy-ass" in his eyes.
The "Belly of the Beast" mission. Jim Raynor, Tychus Findlay, Rory Swann, and Egon Stetmann (the Raiders' mechanic and lab tech, respectively) infiltrate the Nydus tunnel systems on the zerg homeworld by themselves and proceed to tear everything to shreds with a limitless supply of overpowered abilities. One of the achievements is to kill 50 zerg with the Penetrator round (Raynor's special ability). Said ability is on a 15-second cooldown, with no other limitations. The only challenge is getting them to line up for you because he is perfectly capable of doing it if they oblige him.
And after all that, Fire and Fury. Especially Raynor's Rousing Speech, which everyone in the army happens to hear over the radio. Becomes doubly awesome when you consider that what inspires Raynor to make this speech is the dogtag for "M. Koiter", a reference to a Blizzard employee working on the game who died during development. His speech isn't just for his men, it's for Blizzard too—after all the years of trying to match or even surpass the success of one of the most famous RTS games of all time, they did it. They made Starcraft II, it got released, and it was a huge hit like they had hoped.
Raynor:That thing may be the key to stopping the Queen of Blades, but it's our sweat and blood that'll make it happen. After everything we've been through, past all the fire and fury, the one thing I know, is that we can count on each other, to get the job done. Or die trying. If that's what it takes. Because some things are just worth fighting for.
It's even more awesome when you know the Terrans track record from the original game: They never accomplished anything positive without being hand held by the Protoss and were generally only good at making everything much worse. This is the point where the entire Terran faction finally becomes competent on the galactic stage, and a force even the almighty Zerg needs to worry about.
The final mission of Wings of Liberty, "All In", has your plucky band of Terrans and what's left of half the dominion militaryHold the Line for 30 minutes against a non-stop flood of Zerg against the backdrop of the burned world of Char. All of your turtling and base building skills are put to the test. You have multiple fronts to maintain and defend against waves of fliers or endless waves of nydus worm spawned swarms. You have a panic button on a 3:30 timer that non-veteran players (or even veterans on higher difficulty) WILL use as early and often as possible just to get a breather. You even get attacked by the Queen of Blades herself from time to time. At the climax, the device goes off, annihalating Kerrigan's army and landing the underdogs the victory. After the depressing aftermath of the final mission in Brood War, winning does feel pretty damn satisfying
All of this culminating in a Crowning Moment Of Awesome and a Crowning Moment of Heartwarming with the final shots featuring EPIC cinematography make this level the best: Jim blocking Tychus' shot to save Kerrigan, then spinning around and giving Tychus a facelift-by-bullet. And you thought Tychus looked ugly before...
Making this even more awesome is this is the only battle the Queen of Blades has ever lost. Nobody before or after has walked away from an encounter with her with anything but either a crushing defeat or Cruel Mercy.