I loved playing Assassin's Creed II, and Assassin's Creed (original) was my first ever PS3 game, so it holds a special place in my heart.
While I loved Black Flag, it wasn't an Assassin Game. It was a pirate game with some stealth. If it had just been marketed as a pirate game, without the modern day crap they try to use to tie everything together (Hint: They don't need to!) then it would have been my top choice, easily.
More recently, I hated Unity but love Syndicate - the addition of the grappling hook, paired with Unity's new climbing mechanic, makes it a joy to play - plus it's set in London, where I've always wanted to see a game set.
Vote goes to Assassin's Creed II, but it was a close one.