I still have to go with Azure Dreams, an old PS1 game. It was never really popular, but I wasted hundreds of hours of my life on that game. I still pull it out and play it sometimes, and it's still fun.
Oh my god, I love Azure Dreams!!!
I remember spending an utterly ridiculous number of hours playing that game! Hell, I almost managed to get one of every monster, except those damned Picket (was that what they were called? The little thieving ones) eggs always eluded me >_<