Mine was the transportation to Rapture. Even before the game had come out, I'd guessed you would catch a half-second glimpse of Rapture through a tear or something, but they went and put you in the beginning area of BioShock. I literally yelled "HOLY FUCK YES!" when you were warped in. As a biology major, I'd always loved the original's themes of scientific experimentation without morals, and getting a little taste of that again made me very happy.
It makes me wish there'd be a patch for consoles making the combat from BioShock like 2 and Infinite, but it's probably not a good financial decision.