I think Mario Bros.
Deluxe is on of the best games on Game Boy Color. The best thing about it is that it's not too easy but not too hard. You can unlock special levels and worlds. After you beat the first original levels then you unlock the special star levels. After you beat those then you unlock the "lost levels". It's basically never ending! There's also "challenge" mode were you you have to collect Yoshi eggs, get high scores, and collect the five red coins in each level. This game is great for anyone at any age.
