Rez (Dreamcast, PS2)
Hardcore gamers often like to blot out the outside world. This isn't necessarily a bad thing - escapism is what many games are for, after all. Some people on the net talk of their own sound and light-proofed games rooms. While that may be seen as a step too far by some, it would be the ideal atmosphere for Rez.
Set in wireframe 3D space, you control a pulsing object which evolves into human form, eventually assuming the lotus position in the ultimate state of zen as you fly forwards, shooting organic and mathematic shapes in time to the music. The trance-like gameplay and throbbing controller (and its rather dubious optional vibrating "peripheral") will suck you in. And if you're playing with headphones, you'll be lost in a world of color and sound. Eyes wide open, pupils dilated, mouth open in awe.
Its lock-on gameplay may be the same as Panzer Dragoon at heart, and it's not massively difficult (although maintaining a 100% hit rate is seriously challenging), but it is superb. It's available on PS2, or you could seek out the ultra-rare Dreamcast original if you want to be truly hardcore. That's the next thing of course - not just playing it, but owning a mint, original copy of it. But that's for another feature.