Definitely the second Assassins Creed game. MW2 will be more of the same from infinity ward - they make quality products.. but it is kind of the *same* product. As for L4D2, they could have just done a DLC pack or expansion - from what I've seen, there's very little *new* in it.
AC2 will likely have very similar gameplay to the first, but out of the three I think it's the most likely to benefit from a change of venue, it's in a different time period, and the story will likely be a welcome addition; whereas the story in the other two will be minimal at best I would think.
I will tell you this. Call of Duty: Modern Warfare 2 is the best shooter I have played (don't say "it hasn't come out". I played the demo at E3). It does justice to the awesomeness of the first, and it is worth waiting for. I have it pre-ordered.