What would you consider one of the great Xbox games?

I would like everybody to post here what games they have enjoyed the most. Whether it is the more popular games or one of those rare gems that you have uncovered.
Personally I haven't found any rare games I have liked but I know my favourite games for the Xbox are the Fallout and Fable series.
For me they are what a game should be like. Great story with lots of content.
I'd like to hear your opinions.
Thanks