Half Life 2. Graphics were absolutely amazing for their time, it introduced a physics system, the stages felt so much more alive, and overall it's still an amazing experience.
Ditto for Metal Gear Solid 2. Better AI, enemies reacted to wounds on specific parts of the body, destructible things in the environment, dragging bodies and putting them in lockers, among other things.
Playing: Half Life 2, Dark Souls 2, Vampire The Masquerade, System Shock 2, Borderlands 2, Mass Effect 2 PSN: T-Snake94, XBL: T-Snake 94, Steam ID: T_Snake94