I have to say the best computer-game I ever played is an old Microprose RPG called Darklands. It is definately the Ultimate RPG of All Time, in all aspects: gameplay, storyline, the historical value, the battles, the missions ... it never got old or tiring. I recommend that everyone play the game. It is incredibly old by today's standarts, but it's utterly addicting.
After that, I also enjoyed the Baldur's Gate Series, Freelancer (of course), Diablo...