Definitely a hard task to accomplish but I would have to say
Grand Theft Auto San Andreas (Playstation 2) This is the Gold Standard for Sandbox games on the Playstation 2
Kingdom Hearts (Playstation 2) Spend countless hours growing up playing this and has a special place for me.
Shin Megami Tensei: Digital Devil Saga (Playstation 2) Dark and thought provoking as most Shin Megami Tensei games do.
Red Alert 2 (PC) This has fond memories for me as my dad and I Would spend hours together taking on others together.
Rogue Galaxy (Playstation 2) A beautiful game with characters i cared about in a open world that was very vibrant.