well ive played a lot of games going back from the c64, n64, speccy, PS1, megadrive, amiga, various PCs..
It really is quite difficult to pick a greatest because a game at the time can be Legendary.
e.g. Elite just ****** awesome. The era in which the game comes out makes a difference as well....maybe coz gaming was growing but not like nowadays..ie lots of small bedroom/basement devs and such.
A toss up between
Elite
Planescape Torment (pc)
Sensible Soccer (amiga)
Heavy Rain (ps3)
KOTOR (pc)
Monkey Island 2: Le Chucks Revenge (pc)
FF7 (console)
Wing Commander 3 (pc)
Vampire the Masquerade: Bloodlines
Deus Ex
not many new games i guess just coz it takes a lot more to impress me or blow me away these days.
Others which almost make it:
Zelda OOT, Morrowind, Ultima VII, F19 Stealth Fighter, LCP, Edge of Chaos, Dragon Age Origins, Neverwinter Nights, Dune 2, The Nomad Soul, Death Knights of Krynn, Syndicate, Baldurs Gate 2, Fallout 2, Championship Manager 93/94