Pokemon - Crystal Version (USA, Europe) ROM

It's the summer of 1998. The last thing you saw in the first episode was Ho-Oh. And before you knew it, Pokemon Gold and Silver were announced. They were released and were a huge hit! And then came Crystal.
This game was the first to feature animated Pokemon on a handheld device. It also improved most graphics such as remaking Entei and Raikou's sprites to match their artwork, and fixing most sprite related bugs. This game also introduced to option to play as a girl which would be featured in many games to come.
Overall this is a great game. I'd reccomend it to any pokemon fan. 
 

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!

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.

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.