I would have to make mine more of a game franchise. Which would be Battlefield, but if I must narrow it down to a single game it would be Battlefield 2. One of the best online games ever, in the top 3 at the least.
EVE Online. Since Dust514 got announced as changing to a PS3 exclusive I've been a bit saddened seeing as I only have a (broken anyway) xbox360. Not that Dust514 will probably be all that enjoyable to play from South Africa, but still...
I'm going with the FIFA series. I know its had its flaws, etc. Bt the amount of time its provided me with entertainment and memories over the pass 13 years is unsurpassed.
My favourite game of all time is Diablo II Lord of Destruction.
When Diablo II first came out it lacked something and had to be played at a low resolution. Then LoD came out and suddenly Diablo II was awesome. And I just couldn't stop playing. I played it for years. If Diablo III is only nearly as good as Diablo II then it's still going to be amazing.