Batman: Arkham Asylum
Just played the PC demo, and as much hype as there is surrounding Arkham Asylum, I was still impressed. The story and cinematics are fucking cool - the flowing combat engine is a blast - and the stealth gameplay is surprisingly robust, even though I am still terrible at stealth games. With solid mechanics like these, the rest of the game (which includes boss fights with a whole crew of classic Batman supervillains) can't help but be awesome.
Later on, I watched a buddy play the demo on a 360. The demos are the same, but he did a bunch of stuff totally different from how I did it. That's one of the coolest things about the combat and stealth engines; there are so many possibilities, I'm really not worried about them getting old.
Progress: Finished the PC demo