This is an impossible question. Seven Cities of Gold was amazing for the C64. One of the first procedural generated worlds, random events and it all fit on two 5.25" floppies. Zork is a grand game as well, yet it would not even get a spot on Newgrounds today. Empire Was one of the first DOS games that had a workable multiplayer system, and it ran in color on a CGA CRT! Half Life still stands up as an amazing story driven shooter. The Kings Quest series still has a dedicated fan base that plays the game now 20+ years later.

Deus Ex is an amazing game that had an incredible replay value. The GOTY edition fixed some of the original quirks with the control system and fixed some of the graphics issues and I STRONGLY urge anyone who likes video games to play it. GOG link. It is $2.50 people

Is it better than the original Bioshock? Or Mass Effect? Or even any of the incredible console games that have been released? There is a reason Deus Ex is on every "Greatest Games" list, but is it the greatest ever? Does that question even makes sense when art and artistic interpretation are involved?