I think it's funeral doom and atmospheric black metal :).
METAL GEAR SOLID MAIN THEME (Soundtrack - Various Artists, METAL GEAR SOLID) * METAL GEAR SOLID MAIN THEME (1997 E3 EDIT) (Soundtrack - Various Artists, METAL GEAR SOLID) * Metal Gear Solid Main Theme (Soundtrack - Various Artists, Metal Gear Solid - The Twin Snakes) * Title Theme (Soundtrack - Various Artists, Metal Gear Solid - The Twin Snakes) * Armory Encounter Ending Theme (Soundtrack - Various Artists, Metal Gear Solid - The Twin Snakes) * METAL GEAR SOLID Main Theme (METAL GEAR SOLID 3 version) (KCE Japan Sound Team) (Soundtrack - Various Artists, Metal Gear Solid 3 - Snake Eater OST CD1) * Sad Man's Theme (Norihiko Hibino) (Soundtrack - Various Artists, Metal Gear Solid: Portable Ops Original Soundtrack)