The best in the series, hands down. Be playing for a week and still have lots to do, only 50% into the game. Got the Templar Armor and now i'm aiming for all the treasure maps + special outfits.
Did you get all the Templar keys? Mayan artifacts? Upgraded your ship to the max? Got all 22 treasure maps? Crafted all hunting outfits? Got the Assassin outfit by completing 50% of all Assassin missions? Fought all Legendary ships? If no, go play, you didn't "finish" it!
I know i'm taking my time, don't want it to finish just yet. And surprisingly i haven't felt bored by doing those "side mission" things. It actually feels pretty rewarding.