My top five list, in no particular order:
1. Rogue (PC) -- you are a smiley face, a Troll is a "T"...how much better can a dungeon crawler be, pre-GUI?
2. Tron Deadly Discs (Intellivision) -- my grandfather and I would play this till the console got so hot it froze.
3. Grand Theft Auto: San Andreas (PS2) -- can't say enough about this game. I could play it forever!
4. Uncharted Waters (SNES) -- best world exploration game on a console, EVAR.
5. Might and Magic series (PC) -- alright, this is a series, but it's like such a major influence in my life I'm counting the whole thing as one game. ;-)