Picking the best game ever is really a tricky thing. Mostly because every person has a different perception of games that differs over time.
But since you brought up X-Com, yes, that's definitely a contender for best game ever. In my own opinion, the best game would be one that will be played decades after it's release and that attracts fans for different reasons: story, gameplay, design, atmosphere, characters, levels, even grapchis(I still consider that Daggerfall has the best horizont ever and that Settlers 2 looks better than the remake).
A perfectly objective viewpoint to what is the best game ever is completly impossible.
That being said, Anachronox and since fall 2010, Elemental War of Magic.