For ps2? Probably Silent Hill 3. I recall being really impressed by the water effects for Metal Gear Solid 2, so I'd save MGS2 and Snake Eater as well.
PSX? Probably Valkyrie Profile or Legend of Mana. Metal Gear Solid had really awesome graphics too for its time.
PS3? Both Demon and Dark Souls. Ace Combat: Assault Horizon had some really great graphics and effects too. MGS4 looked really good but I've yet to play it.