I've been trying to figure out which game to play next on the ModRetro. So far among Tetris, Toki Tori, and Tales of Monsterland, Toki Tori was probably my favorite. I was worried about the slowness, because these kinds of puzzle games are a lot like programming and Toki Tori definitely felt like that, but I can't help but have an affinity for programming-like things. The game was also just really, really well made. It's written in assembly code and it felt like it - a great economy of game elements.
Wario Land 3 is a platform puzzle game and is pretty widely regarded as one of the greatest games of all time (so far as I can tell) - introducing a lot of gameplay concepts that didn't previously exist. I'm hoping, as billed, it strikes the right balance between platformer and puzzler.
I was a Sega Game Gear kid (basically the Zune hipsters of the day), so I haven't played most of the Game Boy carts and get to enjoy them for the first time. I also plan on picking up The Legend of Zelda: Link's Awakening DX and Metal Gear Solid eventually (MGS is apparently super rare ... carts go for >$100).
Do you have a favorite Game Boy or Game Boy Color game?