The best games really depends on your point of view. Some people say Halo 2 is one of the best, while others say that it was tweaked too much and the original Halo is better. Of course, Xbox Live is nothing without Halo 2.
Just pick your favorite genres and visit some sites that review games and see what they say. Personally, I really like the Chronicles of Riddick, the Burnout series, the Splinter Cell series, and Halo / Halo 2. Fable: the Lost Chapters has me playing more than I should, too.