Well, I think it would be FM - Football Manager. I'm currently playing Football Manager 08, used to be a fan of the series Championship Manager in the old days - ah, I miss the old times. Lucky, the layout has not been changed and still has a bit of the old school feel, though I can't stand the slow loading times sometimes. But overall, the depth of play and AI is good and the level of difficulty provides challenging play for me, I guess.