The whole 'calculation' thing with Ayanokōji is so overdone, but there's one line I come back to. In the anime's second season, after he manipulates the exam results, he says something like, 'I merely arranged for an outcome where everyone's desires aligned.' It's not flashy, but it's chilling. It shows how he sees people as puzzle pieces to be moved, not as individuals.
A lot of fans go for the more aggressive quotes, like his 'tool' monologue. But for me, the quiet, declarative ones hit harder because they're delivered so flatly. He's not trying to sound cool; he's stating a fact about the world as he sees it. That lack of emotional inflection is what makes it iconic for his character.
I honestly think some of his best lines are buried in the light novel volumes, though. There's a part where he reflects on the white room and says, 'Freedom is just another variable to be controlled.' That messed me up for a while.