Well, me being on limited gaming budget, I have to choose between games. And to me... climbing Notre Dame>Pirate ships, hehehe (I'm the sort of person who has looked at buildings since birth, figuring out hypothetical climbing paths, though I rarely get to test them - and yes, I also enjoy rock-climbing, when I get the opportunity). Also Unity has co-op for up to four people. I find that fun.
But, if I ever do get a windfall of money to spend on gaming, Black Flag is on the (very long) list. Because pirates are really interesting, and I would like to captain a pirate crew.