I liked what I played of Assassin's Creed IV: Black Flag but it sort of dragged on for me and I really don't enjoy the sailing, so I ended up abandoning it and watching the rest of the story on YouTube.

This game plays pretty well. They have all the tools established and the island environments lend themselves well to the stealth-combat arenas Assassin's Creed likes to employ. There isn't a ton that is new or stands out in comparison to past games, but among the Assassin's Creed 3 era of these games (that I have played) I think this one has the best execution.
I didn't like the sailing in AC3 and I still don't like it here, but it is put much more front and center, which I found to be a bummer. It always seems to boil down to sailing in circles and having a more powerful ship rather than doing anything interesting.

I like the narrative here. We follow a pirate named Edward Kenway as he comes into conflict with the Templars, tracks down some ancient super advanced device, and eventually becomes an Assassin. He spends all his time with pirates, so there is plenty of opportunity for mayhem and betrayal, most of which is predictable, but fun. However, things do sort of go on for a bit too long, with one or two too many instances of "oh no! Edward was left for dead!" This is the main reason I lost interest and abandoned the game and I would have liked it more if it were about half the length, I think.
Kenway is an interesting, morally grey character who has a bit more agency and personal motivation than I am used to with these games. He is sort of out for his own benefit the whole game and has some real character growth into becoming an important part of the Assassins. The protagonists that are less of a blank slate are just more interesting in this series, I think.

Black Flag is a bit long and has a bit too much sailing, but it is a solid entry that explores a cool part of the world and tells an interesting story.