The best game for PS2 in my opinion was MGS3, it's also one of the best PS3 games.
Oh yeah, as to why: huge replayability factor thanks to unlockables, great Cold War espionage story, loads of intricate gameplay mechanics such as the stamina bar which is replenished by eating (but eating certain things can poison you, make you ill or kill you), camouflage index, hold ups and CQC (allowing you to use people as human shields and interrogate them)... That list goes on. It's also got decent gunplay but is far better if you avoid it and the graphics (and possibly sound) were the best I've ever seen on the console. Five difficulty levels meant anyone could play it and have fun, too. Oh, and the crocodile cap.