At the moment, I'm currently playing a game that isn't even considered mainstream for the sake of beating it. And that game is Harry Potter and the Chamber of Secrets for the Nintendo GameCube.
Despite being loosely based on the book of the same name, the game plays a bit like 3D Zelda installments. I will admit that I'm not a fan of Harry Potter, but I feel it's actually an above average license-based game of its generation. It's still fun to explore the areas even if they are a bit constrained. Though, occasional camera problems, unpolished targeting system and overuse of (very easy) stealth sections are what prevented the game to become a true hidden gem. Thanks goodness the game is very forgiving, as dying isn't even painful at all and that you have a potion that will refill your health bar as soon as you run out of life.
For the record, I'm only on Day 1 of Hogwarts, already completed the Burrow, Diagon Alley and the fight against a huge living tree. From the looks of it, I'm about to have some lessons on how to fly with a broom.