Bear in mind that the following recommendation comes with a slight bias. I've been a steadfast fan of Assassin’s Creed ever since the series began. Every game has been a unique take on the central concept of “Kill people in exotic historical setting.”
Assassin’s Creed 4: Black Flag moves the setting back from Revolutionary America to the high seas circa the early 1700s. The main character is Edward Kenway, grandfather to Connor, the star of AC3. Little to no mention is made of the other heroes of the series, leaving the spotlight focused very narrowly on Edward.
I found so much to like in this game, even more than the raw schadenfreude of being able to stick someone in the ribs with a switchblade attached to your wrist after you've utterly destroyed their ship with cannon fire. Edward is a well-written anti-hero. Every location in the Caribbean is beautiful. Even the modern day setting, in the office of the person researching Edward’s life, is gorgeously animated.
The characters are likable almost in spite of themselves. Just like real pirates, not a single one of them is a purely good person, but it’s hard not to sympathize when one sees the circumstances they are working against. They are just trying to get by, not rolling in gold like the hedonistic pirate captains we’re used to seeing in fiction.
For once, a main character is allowed to question the Assassins; unlike Ezio and Connor, Edward does not agree blindly with their core philosophy and initially only works for them because they provide steady money. I won’t say any more for fear of spoiling the main story, but I enjoyed the way the game played with the usual Assassin vs. Templar dynamics.
Assassin’s Creed 4 isn't the best game of the year, or in the series. However, it does provide a story you won’t find in any other game out right now. Oh, and prepare to have sea shanties stuck in your head for weeks. “I’ll sing you a song of the fish in the sea,” indeed!