In terms of a sense of actual accomplishment, probably Frequency on PS2. I worked hard to hone the skills necessary to beat that game, and I don't normally bother working hard at games.
In terms of satisfaction, I'd have to say Final Fantasy VIII, it was a long journey with a conclusion to the story that totally blew me away, being the incurable romantic that I am.
In terms of satisfaction, I'd have to say Final Fantasy VIII, it was a long journey with a conclusion to the story that totally blew me away, being the incurable romantic that I am.