so hard to choose ONE....
God Of War... Uncharted... ICO... MediEvil.... Shadows of Colossus... Little Big Planet... inFamous.... Killzone... Twisted Metal... Wild Arms... Legend of Dragoon... Journey.... (...) so many epic games.
i choose the masterpiece... Uncharted
Probably will have to be either the "Uncharted" series, or even the "God of War" series, or even the "Infamous" series, or what about the "Medievil" series. I can't say MGS cause there are sequels and other related games to other systems. Maybe even the Original 3 Resident Evil titles they had, that's my answer. The "Resident Evil" Trilogy for the PSone. If it wasn't for those games, we wouldn't have such a outcry about the next couple of Resident Evil games that came out afterwards.