This is a really good question, it has certainly got me thinking.
It's such a tough choice. For me it's between:
Elder Scrolls IV Oblivion - I don't think it's great but it has a phenomenal amount of content.
Zelda Wind Waker - Almost Endlessly charming, almost a true open world with plenty of interesting content and a great aesthetic. However, not enough content.
Bully - A fantastic game, thoroughly entertaining. Poor graphics and limited gameplay make it a loser though.
Grand Theft Auto IV - A massive game with lots of content. The sandbox world allows you to make you own fun for hours on end. The city feels alive and hugely detailed. Considering the DLC coming out it's a serious contender for the winner.
WOW - Never really played it but it seems to have enough content for around 3 years of uninterrupted gameplay.
Battlefield Bad Company - A brilliant game in which the multiplayer has incredible replay value. I must have played this games for 40 hours or so and I keep going back.
And the winnder is:
GTA IV - The amount of content, the varied gameplay and the vast range of characters make it a winner. If you consider the multiplayer you add even more dynamic towards the title.