A Big Daddy giving Suchong his awesomely appropriate Karmic Death.
Steinman, before the boss battle with him. He's in the middle of doing his "surgery" on an hapless splicer when he notices you. His response? Instead of cowering behind some elite minions like most bosses in video games do, he gets pissed, picks up a tommy gun and fires at the window you're looking through.
Holding off an army of Ryan's splicers while the Lazarus Vector cooks in Langham's lab.
Followed by The Reveal that Frank Fontaine's was the mastermind behind the entire thing definitely counts as well, and instantly seals his spot as a Magnificent Bastard. While many such Bastards are indeed Magnificent, not very many have sold themselves as such within the first minute of their entry into the story. Just three words: "Would you kindly?"
The part where he pretends that his family is aboard a bathysphere that Ryan blows up, just to fuel the player's motivation against Ryan.
Except Ryan didn't even acknowledge the sub or any of its passengers. He only states his intention to kill you, a key pawn in Frank Fontaine's Evil Plan, and it is Atlas, Fontaine's alias, who was last seen fiddling with the sub controls. Fontaine more likely rigged the sub to explode beforehand in preparation for Jack's return to Rapture. He is a Magnificent Bastard, after all.
Setting Sander Cohen's Quadytch on fire after finishing it. And, when Cohen goes ballistic, killing him and getting the Irony achievement for snapping a picture of his corpse.
Except not really, because the entire game is about how badly that choice went.
Try looking just up above the first Little Sister you can save or harvest. "The great shall not be constrained by the small," indeed...
The scene where Sander Cohen thinks you're mocking his masterpiece and sends several Splicers after you while Waltz of the Flowers plays. It's such an incredible scene. Seriously, try not to be amazed when you whack a Splicer and they go twirling off to the side as if it's some grotesque ballet. And the fact that Sander shouts out "Fly, little moth. Fly!" and "Smile! SMILE!" like it really IS a grotesque ballet makes it all the more better.
It is possible to engineer a situation in which two Big Daddies fight to the death while a Little Sister is standing in the background shouting "Hit him, Mr. B!" Doing requires that you die with a vita-chamber close enough for you to run back and watch the action after your resurrection, but it's well worth it.
Or save enough Little Sisters to get the Hypnotize Big Daddy plasmid, use it on one Daddy, then find a second with a Little Sister, shoot the Sister's BD with one bullet, and watch the chaos.
The confrontation with Andrew Ryan is, in a very meta way, one for the makers of the game. On one level, the Player Character is confronted with the truth that they are in no way in control of their actions, but are being made to do whatever Fontaine/Atlas tells them to, due to three simple words. But on the other hand, it is a Take That directly to the player himself, as he has, up to that point, followed every single instruction given, without questioning whether or not they had a choice... without realizing that the Player Character literally didn't have a choice. In other words, since the game began, you've willingly made yourself the slave of the person giving you instructions, without ever questioning why you were doing these things.