Well deciding what the greatest Rpg made is impossible to do (obviously

) but I think these are the main answers you'll get:
Planescape: Torment
Deus Ex
KOTOR (and arguably KOTOR 2, though a lot of people think KOTOR 2 was a bit disappointing)
Dragon Age: Origins
Baldurs Gate 1 & 2
Skyrim + any of Bethesda's sandbox Rpgs
Fallout 1 & 2
The Mass Effect series
I've probably missed out one or two games by the way (some of them are probably games I love too, so don't take offense) but those are the top contenders.
EDIT: I didn't add Jrpgs because...well not to be offensive but are Jrpgs really even proper Rpgs? I haven't played Final Fantasy, Persona or Chronotrigger though so I might be wrong. I've heard though that you don't even role play a character in those games and you apparently don't even get to choose how you level up and etc.