Here are my favorites:-
BioShock
BioShock is a great game. It is the best game that the Xbox brand, not just the Xbox 360, has ever seen, and will likely go down as one of the best games ever on any system. It is one of those rare games that combines not only amazingly polished graphics and sound, but very solid, deep, complex gameplay, a great story, and an overall feeling of immersion that sucks you in and keeps you playing that few other titles can match. BioShock is a must play game for Xbox 360 owners.

No Multiplayer mode though !!
Assassin's Creed
Assassin’s Creed is one of the most intriguing, fresh, fun, and absolutely impressive games to come to the Xbox 360. The presentation is stunning and the gameplay will hook you and not let you go for hours at a time. It is not a perfect game by any means, but when you take your time to really experience everything the game has to offer rather than rushing through it, the flaws fade and the strengths shine even brighter. Assassin’s Creed is a great game that is among the very best of 2007.
Fight Night Round 3
Fight Night Round 2 was one of our favorite Xbox games of 2005 so Round 3 had some pretty high expectations to meet. Amazingly, EA has delivered a boxing experience that easily surpasses everything that Round 2 had to offer. The gameplay is absolutely phenomenal and the graphics on the Xbox 360 are amazing. Not only is this the best boxing game ever but it is also one of the best Xbox 360 games available.
Dead Rising
Recommending Dead Rising is easy based on your reaction to the following statement: All you do in Dead Rising is run around a mall and kill thousands of zombies in hundreds of different ways. If you first thought is “Wow, that will get repetitive after a while”, you should probably rent it. If, however, your first thought is “That is everything I ever dreamed of and wished for in a zombie game”, then Dead Rising is worth a purchase. It really is as simple as that.