#4 Puzzle Quest
Skip the sequels and the offshoot and just play the original Puzzle Quest. Ageless, timeless, or whatever you want to call it, the game takes the concept of Bejeweled and embeds it in an RPG that'll have you playing the game for hours upon hours on the train, in class, and just about everywhere else.
The game sinks its hooks (in the form of RPG elements) into your flesh, challenges your mind with its devious puzzles, and refuses to let go until nature demands that you sleep, eat, or do something otherwise more productive than playing Puzzle Quest.