AC Black Flag looks nice but after the novelty of piloting your ship wears off there is nothing very interesting in this game.
Removing the x button to initiate climbing ruins running around as you always climb a wall or jump onto a pillar that you don't want to. The characters and story are forgettable and the rest is go here kill someone or open a chest.
The old AC gameplay just feels tired and boring and putting in ships only masks the underlying problem that this franchise needs to go down to Davey Jones' Locker
As the title implies, it's definitely the best. Most of the game will be spent on your ship, especially so if you want to complete all the side missions and ship upgrades. The ship controls very smoothly, and isn't plagued by he terrible controls that the AC series is known for when on land.
Plenty to do when you're bored of assassinating. Smuggler's dens, underwater ship wrecks, naval missions and more are all available to complete.
There are only a few downsides to the game. One of them being the story moves at a snail pace and takes a long time to actually get to anything interesting. The characters are all likeable but it spends too much time on menial details. That said, this is perhaps to add some depth to Edward since Ubisoft will more than likely continue Edward's story with a sequel or two. Speaking of Edward, I found him to be the most likeable Assassin yet! Has as much personality as Ezio, if not more, without the brattiness.
Overall, the game does just about everything right with minimal complaints. Only real downside is the do-everything-with-one-button which leads to you doing things you didn't want to do various times.
Side note: Ps3 version runs surprisingly well with some frame drops in some parts of the cities. 1080i too.