Metroid Prime. Super Smash Bros. Melee was a very close second.

I would also go with Tales of Symphonia. In my opinion, it was a bad gen for RPGs; this game was one of the few exceptions.

Three way tie between Star Wars Knights Of The Old Republic, Final Fantasy X and Halo.

If you do not count PC games easily God of War. There may be other "deeper" games. But not one being more fun or impressive.

I don't know if it was the best, but I probably played SSX3 on the PS2 more than any other game. Infinitely replayable game. On Tour and Blur have been tremendous disappointments since.

I say that as a person who doesn't really care much for racing games or x-treme sports. That a game that combines the two held my interest for 100 hours or more is saying something.

XENOSAGA 3

For me it was GTA San Andreas. Everything I loved in the previous GTA plus everything I wanted and more. Spent easily over 100 hours on it, doing missions and everything plus whats more fun than a sniper rifle and a roof :D

By far , hands down God of War 2 . It was nearly perfect in every way . The graphics are still breathtaking , epic feeling untill the end, with huge landscapes the game is much longer than the first one , the ending lets you want more , but not in a bad way , and the story is absolutly phenomenal .

Also there is POP : SOT , but quality wise GoW2 is superior ...

Super Smash Bros. Melee

 Melee for me aswell