The truth: No matter what they say, they're all pretty much the same :)
Your answer: You tell them what they want to hear.
On each of their websites, they list out their culture. Reference some of those keywords in your answer!
Importantly, most will say they value diverse teams/teamwork, collaboration, curiosity, hardwork, drive, etc. It's ok to have the same answer for all of them, as long as they all purport to value those aspects!
Also important: You are not jsut saying why you want them, but why they want you So, if you say "I like Parthenon because they value diverse teams and topics", also say "This is how I've worked my whole life across x important project and y important initiative".