wow..that's a difficult one to answer because there have been quite a few. if i were to pick one i'd say Mass Effect 1/2 amongst the newer ones..amongst the older ones i'd say System Shock 2.
[Edited by saurabhfzd, 3/25/2011 6:16:39 AM]
The best game ever should be a game that's always fun to play, with lots of missions/quests where just one answer or action could change anything later in the game, a game which would constantly generate new missions/quests, new actions, surroundings, dialogs and perhaps even the timeline of the game. Something like a third person sandbox game with a lot more decisions and options than the games currently available to us, imo.
But if i would have to choose a recent game i'd say "Mass Effect 1 and 2" because of the amazing Story and graphics.
I like games and movies which has good content, good gameplay. Not only graphics/special effects showoff which is nice of course but meaningless without good gameplay/content in it.
Followed closely by Total Annihilation