Myriad is right.
Prince of Persia: Warrior Within is one of my all time favourite games on the PS2. It took the series in such a radical direction from the first game, and has one of the best combat systems that I've ever used, far surpassing something like God of War. The soundtrack is insane, the story is great and the game mechanics and features using time travel make it an awesome awesome game.
As for Contra, I had the misfortune of being born past the hey day of the NES, and thus couldn't experience the glory of Contra and the animalistic muscle memory of the Contra Code. That being said, it is an immensely enjoyable shooter that is a great way to kill time with friends, a worthy opponent for any game.
However, PoP gets my vote.