Getting the Summon Eleanor Plasmid. The weak, frail girl you've been trying to save throughout most of the game is now a Big Sister who can rip enemies to shreds, taunting them the whole time.
Even better, as far as the splicers are concerned, this is their Messiah personally killing them while mocking their religion.
Anything Eleanor says while fighting as a Big Sister.
"Stay away from my father!" "This one died alone and afraid! Stand in our way, and you'll get the same!" "Just try to stop me protecting my sisters. Please, try!"
Eleanor uses telekinesis, yanking a splicer across the room onto her needle! What does she say?
"I am not your bloody Messiah!"
Learning the 'Drill Charge' technique. Remember the Bullfight Boss that the Bouncer Big Daddies were in the first game? Well, now you're the bull, and the Splicers aren't as good at dodging as you were.
The Handyman tonic. Suddenly your hacked security bots turn into true bros who lay their very lives on the line for you.
If you let Grace Holloway live, she realises that you're more than an unthinking monster and decides to help you out - and, as Subject Delta massacres his way out of her HQ, accompanied by a swarm of donated security bots, blasts a full-on rendition of her signature hit over the PA system. Looks like someone's had a weight lifted off their mind...
The evil ending. Ruling the goddamn world together as father and daughter? Shit yeah! Daughter, I am not disappoint. (Not to be confused with the pure evil ending where Eleanor takes Delta's ADAM against his will).
"Meltzer's Choice", in its own sad way. Mark Meltzer has been captured by Lamb, and she starts giving her usual "ask yourself" spiel to convince him to either be turned into a Big Daddy or be executed. His response?
Meltzer:*dark laugh* ...I wasn't the first to find Rapture, you crazy bitch. And I won't be the last. You do ... whatever you want to me... as long as I'm with Cindy... I'm ... I'm a happy man.
The flooding of Siren Alley. There's nothing quite as exciting as outrunning an entire flood as pipes burst around you and the place practically falls apart. Probably one of the best moments of 2.
Eleanor Lamb gets one when you learn the full scope of the intricate plan she set up behind her mother's back to resurrect the years-dead Subject Delta (you) and lead him/you to her. And of course, she gets a minor one every time you use the above-mentioned Summon Eleanor Plasmid to call her to the field and watch her wreak utter havoc against your now-very-outmatched enemies. Lastly, in some of the endings, watching her, in the final cutscene, slay her mother.
Do you believe in Laser-Guided Karma? Did you save all the little sisters yet put down Grace, slew Stanley in cold blood, and mercy-killed Gil? Enjoy the utopia you've made with a true family of little sisters, with Sofia Lamb dead at Eleanor's feet.
The ever popular scene concerning the Unstable Teleport Plasmid; this plasmid is completely bonkers. It starts to teleport randomly about the area every time you try to grab it, until it finally settles down, then starts hovering and starts teleporting Delta around as well. This includes a shot of the plasmid chasing down a Splicer. Who. Is. TERRIFIED. FOR. LIFE. Yes, in this game, even a random joke item gets a CMOA.
Destructive Learning, especially considering what Delta has just been through:
Eleanor: Hello, mother. As you've always said, I shouldn't be allowed to wander the city in my condition, but without going outside, it's harder to hear much about the world, so I'm learning at home! This, I think, is a... porcelain tea service, in the style of Louis the 15th, isn't that right? (sound of a crash) Now that's a fascinating sound! And this is called a 'stained glass window,' hand-crafted by your adoring flock ... they've gone to a lot of trouble to capture your likeness, haven't they? (sound of shattering glass)
Gathering ADAM from a certain corpse in Dionysus Park (it's in the room just before the carousel where Mark Meltzer encounters the Big Sister and Stanley Poole contacts you for the first time) will play Sander Cohen's piano theme whilst splicers attacking you, a faithful Call-Back to the original game.
Some cut Audio Logs showed an entirely different ending for Sinclair: He is heading to the lifeboat on the Atlantic Express with some samples of Rapture's genius to sell off topside, when Sofia reveals that her people had snuck a bomb onboard. Defiant, Sinclair releases the train car from the track and falls into the abyss below Persephone, ensuring the bomb will detonate harmlessly...for Delta, anyway.
Sofia: I told you Augustus was a liability to you, Delta. His precious train-car full of loot has a bomb affixed to it by my agents. I was hoping I wouldn't need such a blunt instrument, but... If you proceed, Delta, I will detonate the car as you pass, and you will vanish forever into the abyss below.
Sinclair: No sir. Now, son, you listen. You take young Eleanor to my lifeboat and you get her topside. Doc Lamb is no kind of momma to her. Now I'm anything but a saint...but Lamb has me cold. An' well, I've been watchin' all you...an' young Eleanor have endured. An' perhaps it's havin' to see it all through your eyes an'...if I'm honest, more'n a little o' that hurt was on my account. Anyway. I reckon it's high time we settle up. Please consider this my fare-thee-well.
Now, Doctor Lamb, I know you can hear me. An' I was hopin to see you about a little problem I've got.
I have this powerful fear o' fallin, Doctor, an' I hope you can help me shake it. One thing, though. There's this lever in here labeled "emergency cable release"? An' I hear the pit beneath me's near bottomless. Bomb wouldn't do much harm down there. So, ah...you might have to talk fast.
Using the Gravity Well Plasmid. 'The power of a Black Hole in the palm of your hand.
Porter's response to Lamb's invitation into the Family. He may not have known what Lamb was up to, but damn if it doesn't feel good after what she did.
Porter: Dear Doctor Lamb, I received the invitation to your little social club today. In return, I'd like to make you a wager: I wager you need Rapture Central Computing just a little bit more than I need your half-baked metaphysical mumbo-jumbo. I deal in science, not...whatever it is you're up to! You keep out of Minerva's Den and I'll keep this city's automation from grinding to a complete and sudden halt. Sound fair?
Even better, Lamb's first followers were people with weak wills and those with past traumas, like having lost people close to them. Porter is essentially a prime candidate for her therapy, but rejects her easy way of getting over it.
At some point, someone recommended that Porter use ADAM to turn himself white and get ahead. Porter's response?
Porter: Well, that's some idiocy! I told him, "First of all I AM ahead. Second, in Rapture, it's your WORK that's supposed to matter, not your skin!" Too bad for some folks you can't splice in common sense.
The fact that Porter not only managed to create an artificial intelligence that isn'tevil, but one that can replicate human personalities almost perfectly. Especially when it's revealed that the Thinker was imitating Porter himself the whole time.
Double points for the Thinker acting as The Chessmaster to ensure it and Porter could escape Rapture. It set the entire series of events of the DLC into motion.
Perhaps Porter's greatest achievement? After all the hell he went through, he gets to take down the guy who screwed him over, escape Rapture with Tennenbaum, returns to being human, brings an incredibly advanced Artificial Intelligence to the world, and finally has the chance to move on with his life. Damn right he earns that happy ending.