Hard to say, but I'd probably have to go with good ol'
I've played through the Baldur's Gate series from beginning to end, counting the first game and the expansions, at least three or four times now which doesn't sound all that impressive till you consider the 100+ hours one of those playthrough takes, and that's not even counting the numerous half-way campaign and games where I only played one or the other of the series.
Yet despite having played the game so often I continue to find new things in it, new scenario, new items, new dialogue. Not to mention the modding community which to this day continues to produce amazing new content and improve upon the games solid foundation.
Just such a solid experience, I don't think I'll probably ever play another rpg quite like it. Pity!