Games where you were obsessed with completing it, and had loads of fun doing it.
No1 for me was Tomb Raider 2. The first time i played it i hated it, it took me hours to get past the first part, and then i ran into those dinoursause which scared the hell out of me. After that though i started to enjoy it and it took over my life.
No2 is FF9, the first ff i played, i didn't like that to begin with either, it was the first rpg i played so i didn't understand the whole magic and random battles stuff. When i got to Dali i started enjoying it and had a great time playing it.
No3 is LOZ The Wind Waker. I love the look of it, and the game was really good, not as good as the other two, but it still took over my life for a while.
So what are yours?