I'm saddened that nobody gave props to Planescape: Torment or Fallout. Are there no western RPG fans around here?
I'm glad at least one person enjoyed the original Xcom game.
It's hard to ascertain exactly what "best" is. If by best, you mean the most hours I've played a game, that would have to go to the original Quake and all its mods. That game sucked up almost all my gaming hours between 1995 and 2000, but I credit that to a very active community. That, and I "pwned" in Quake. ;)
As far as the most fun I've had playing a game, that's hard to put a point on. I'm just going to throw out some games that I love to play.
The games mentioned above
Any Squaresoft game pre-FF8, sans Mystic Quest
Pac Man
Any Zelda game besides Majora's Mask
Ico
Super Mario 64
Any of the Mario Karts
Archon
Goldeneye
Rock and Roll Racing
Pilotwings
Gitaroo Man
Grim Fandigo
The Monkey Island series
Zaxxon
Agent USA
In other words, I love games.