Okay, when I was growing up it was a two-way tie. Since all we had was an old Commodore 64, both games come from it - back in the old days when floppy discs really were floppy.
Defender of the Crown: One of the orignal turn-based strategy games you could play, and despite having a somewhat corny plotline and being pretty predictable, it was strangely addictive. I loved it.
The Last Ninja: Much more in-depth game, several levels each with their own challenges and weapons to collect, and little secrets that you sometimes had to find and sometimes just found as a bonus. Never could win it, though... the final boss always beat me before I could figure out the weird orb thing.
Both games get an 8/10
In more recent times it'd have to be a three way tie...
Starcraft: RTS game, love it, will never stop loving it. Instead of the two functionally identical teams that you get in most games (Warcraft, Dark Reign, Red Alert, Dune, etc), the three teams were completely different and added whole new balance to each level. And the story was awesome.
Diablo II: This game just kicks ass. I liked the Necromancer best, though the Amazon was pretty fun too. The Druid just pissed me off. But anyway! Graphics are excellent, the level randomiser makes replayability almost infinite, the skill trees mean it requires a lot of planning to win, and the story is magnificent.
Elder Scrolls IV: Oblivion: Do I really need to say anything?
All three get a 10/10