I feel like the Batman stealth was amazing in Asylum, and while it was definitely present in later games, it never seemed as strong. I might be misremembering, but in Asylum, the point of a lot of encounters seemed to be to take everyone out one at a time. In later games, the focus was on freeflow combat and crowd management, less stealth and more action.
I agree that, at its height, Akrham's stealth is better than what you do in any of the Assassin's Creeds I've played. To be fair though, I wonder if that's down to the world and structure of the game. The Akham games, typically have you moving from arena to arena whereas the Assassins games are usually in open environments. If they did trap you in tighter enviroments and rooms occasionally and design encpunters around that, they might be able to create the same effect for players. Honestly, I'd love to see them try.