The Beatmania IIDX series: If you think you know about hard games, you ain't seen NOTHING yet. This game makes MGS3 and Ninja Gaiden Black look pathetic in comparison when it comes to sheer challenge. It will kick your ass at every single step in your skill progression, from your earliest beginnings to when you can AAA some songs with no problems. And best of all, it's such a fun, intuitive series, that you'll keep coming back for more. Significantly easier in the US release, but still VERY hard and VERY fun.