The best football game on the PS2 is Pro Evolution Soccer 5. If you are new to this game series then it is worth persisting with, as on initial play it can be quite frustrating. However, once you have had a little bit of practice you will soon come to realise that this is the most realistic football game on the market for any console.
Some of the earlier versions (PES 4 for example) are available for great prices these days and should also be considered.
Steer clear of the FIFA series – they are nice looking and have full player/team licensing (PES has partial licensing) but ultimately are unrewarding as games.
Pro Evolution Soccer 5 is the best by far.