Baldurs Gate 2 is the most succesful FRP that I ever played. The golden point of the game is interaction and the feeling of adventuring. You save people, help them, rescue their village and they build your statue, even you can have a stronghold you can have taxes you can help your villagers, you can HAVE ROMANCE! and sex of course
This game is huge! But the most side I liked as I said the interaction between npcs and you. Daggerfall was also great, Might and Magic VI was good, Fallout was great and Phantasy Star 4 was the second best FRP that I ever played.