Pokemon - Crystal Version (USA, Europe) ROM

This is without a doubt the best Pokemon version for the Gameboy color!
I came from playing the yellow version and I could not get over the difference. The gameplay is similar enough, but the graphics are far improved, not to mention this game is in the johto region, unlike pokemon yellow. 
Like all of the pokemon games, you play as a trainer and your object is to train pokemon and collect badges. This game is great fun, even for those that aren't pokemon fans. It's sort of like an RPG, but not.

Pokemon Crystal is the polished version of the second installment in the Pokemon series. Being a more complete version of Gold/Silver, it shares a lot of similarities, and is essentially the same game. However, Crystal has an extended story, and some events that were not present in Gold/Silver.
You still play the role of a character from New Bark Town, but this time you're given the option to choose your gender. You set out on your journey the same way, but the story now has a bit more emphasis on the 3 legendary beasts of Johto (specifically Suicune). As usual, the wild Pokemon have changed a bit, and so has the list of ones available. Some Pokemon can even learn different moves than they could in Gold/Silver.
If you're content with Gold/Silver, this game isn't really necessary, but if you haven't gotten either of the Johto region games, this one is recommended.

Travel back to the world of Johto as the Gold & Silver series continues with brand-new features! Whether you're reading the fresh descriptions in your Pokékdex, using the unique sort function to organize your Pokémon in new ways, watching the all-new battle animations, or discovering another way to capture Suicune, you'll need all of your Poké-skills to master Pokémon Crystal on your Game Boy Color!
 

Amazing game, perfect for anyone studying actual transitions in game. Seems like the only breakthrough they've made since has been graphics and running. (nothing for B to do DX).

Pokemon Chrystal is a fantastic RPG for all ages as you embark as a trainer with dreams of becoming the best trainer in the world. This game is a classic and always will be!