Zone of the Enders
Also known as "the disc that shipped with the Metal Gear Solid 2 demo," Zone of the Enders is about as inaccessible to casuals as games can get. Very Japanese in its stylings, it draws heavily on inspiration from manga greats such as Neon Genesis Evangelion, seeing a child thrust into control of a living mech after the death of his family.
Combat with other mechs is beautiful, with plenty of collateral damage to "ooh" and "aah" over. If only the game had a little more variety (and accessibility), it could have achieved star status. Instead, it will sit forever on the hardcore gamer's shelf, remembered fondly by those in the know as an icon of early PS2 gaming and the possibilities of the then next-generation of consoles.