That's funny, I've mentioned my top three a few times over the last month... Otherwise I wouldn't be able to remember what mine are!
Football Manager 2007
Sensible World Of Soccer
The Oddworld Series (if you're going to say "pick one", Abe's Oddysee)
Quite close to call, but at the same time there was nothing else I could have picked. I don't finish 99% of my games, especially now on the 360 where it seems there's a quality title out every bloody week, but FM 2007 I could play every single day until I die and not moan about it (as with every single previous version), Sensible World Of Soccer was played every single day at my house for what must have been over a year (so much to do, so many teams to be, such a challenge finding decent players for your Ecuadorian Premiership team - will we ever see a game with such a deep database again?), and Abe's Oddysee was the first game since Super Mario 2 on the NES that made me sit down and finish it - over and over again (I can't think of any other game I've finished more than once, apart from Abe's Exoddus).
Guitar Hero nearly made it, simply because it's not your average game (if the Wii can make you feel anything like GH then I will be getting one without a doubt),
Winning Eleven 6: FE nearly makes it because my dad, who previously didn't really give a monkeys about games, couldn't comprehend how a game could be so good (so it must have been bloody good), and
Gears Of War nearly made it because, quite simply, it has the best gameplay any shooter has ever seen.