Experienced playing it at Otakon last year and realized that the Dodonpachi series probably has the hardest games on the planet.
It is a shmup game infamous for its difficulty, although not as well known as Touhou. But it is marginally much more difficult.
Anyway, I watched the Black Label version of the title game and saw the True Final Boss (and the the TRUE True Final Boss) which are insanely hard.
Here is a sample of the True Final boss: http://www.youtube.com/watch?v=SVSVrcUW17E
Keep in mind, there is a harder boss after it that can be unlocked a certain way.