Arkham City - 8/10?
I found this one really hard to judge, especially compared to Arkham Asylum.
It felt less focussed to me, without as much variety, and it had less of a focus on the individual villains and their distinctive features - here the villains are the more human ones like Penguin and Two Face, which means most of the game is spent fighting generic thugs and you dont get as much of a feeling that this is a Batman game. In Arkham Asylum there were more scenes of areas where you went into the lair of Killer Croc, Poison Ivy, or the Batcave, etc, which were really distinctive.
So I came out feeling like the story wasnt as strong. Except... the ending was really strong and almost changes my mind. Its kind of like the story hid in the background for most of the game and then jumped out and surprised me. Part of the problem I think is that youre very reactive... you just go from place to place and things happen to or around you.
Gameplay was kind of the same... Heaps of fun, but not quite the same amount of variety as in the first game, imo.
So I figure 8/10, compared to 9/10 for Arkham Asylum. At the end of the day, you still get to feel like Batman.