ICO - Beautiful game whcih really gets you involved with it... could be a bit longer though.
Gran Turismo 4 - Best driving 'game' of all time. The depth and amount of customisation is just unreal. Actually takes some skill to win races aswell unlike NFS etc..
Tekken 5 - Thought I'd mention this because Sean said Tekken 3. Basicly Tekken 5 is Tekken 3 but more intense, quicker, and more addictive.
Metal Gear Solid 3: Subsistence - Great storyline, best graphics on the PS2, alot of replay value, internet play, fun mini games. It's the perfect game.