Off the bat, I'm saying the "new" Batman games, mostly Arkham City. As an avid fan of Batman and one that has the obsessive-compulsive need to collect EVERYTHING, it always gives me a lot to do if I end up starting fresh.
I'll also look a bit into my past with my older games and such and I have to say I find most of my time (if not on my 360) indulging in a bit of nostalgia with Twilight Princess and even Donkey Kong Country. While Twilight Princess has a nice story and good graphics (C'mon, it turned Link into a looker at LEAST), as one who didn't grow up with OoT, I find it a good fallback if I ever wanna play something I know I was good at playing, perhaps because it was easy, or perhaps because I felt that I was actually good at it. Donkey Kong Country, while having little to NO story, is one of my childhood memories and before I even started playing it, I remember watching my mom play it and that game just never gets old. If all else fails, I love the soundtracks to both games and if I DO get bored, I can just listen to the music.
Other honorable mentions are Mass Effect 2(Everything's awesome), Dead Space 2(Love the elements and the direction they took with both female characters in the media and insanity), and Bastion(Beautiful and gripping)