Simon,
It depends on what kind of games you like and how old you are.
In my opinion, God of War is one of the best games on the PS2, but might not be right for younger gamers trying to get their parents to buy them a game.
Good for everyone though is Okami. Really original action game that has some great gameplay. You really can't go wrong with that game. It even got GR's best game of the year award.
http://www.gamerevolution.com/features/best_of_2006/13 Get it, play it, love it.
Okami is truly a beautiful game that makes me wish I wasn't so achievement point obsessed on the 360 so I had time to go back and finish it. Spending too much time playing Burger King games just for the points.
- C!