There's a difference between my favourite game of all time and what I would choose if I could only play one game.
Favourite game of all time, ultimately is probably Planescape:Torment. I've enjoyed other games as much, but haven't been back to any of them (except possibly Ultima 7) as many times as PS:T. I've played it through at least twice just for fun, once making sure I did all the quests and got the bronze sphere, once again to do the candlestick quest and I'm highly likely to put on the Unfinished Business pack and do it again. Even now it still looks and sounds good. Ultima 7 is nice, but it does show its age.
If I had to choose one game to play forever, however, it would probably be Oblivion, or possibly Baldurs Gate 2. I've not played Oblivion for months and I'm not sure why - perhaps it is simply a bit *too* big. The quests are inventive, the interaction impressive and the graphics amazing. Also, both Oblivion and Baldurs Gate 2 have hundreds of mods. You could spend years playing it.
PK