I think under "greatest game of all time", it has to be any game which you've woken up one morning, and just played until bedtime again. And not consider it a day wasted.
I don't do that too often - cos I have a life away from games ... but I'd consider the following under the tag of "gaming classics" which meet that criteria ...
1) Rebelstar (ZX Spectrum)
2) Elite II (Amiga)
3) Space Hulk (Amiga)
4) Secret of Mana (SNES)
5) X: Frontier (PC)
6) TIE Fighter (PC)
7) Star Wars Battlefront 1/2 (PS2)
8) Warhammer 40,000: Dawn of War series (PC)
An overall winner? Has to be the Star Wars Battlefront series. It's just probably the best game ever made, loads of different ways to play each level, fast, frantic, deadly.
You play the whole game as a Stormtrooper - it gets boring. So you then try playing as a Dark Trooper, it's a whole new game all of a sudden.