Wario Land - Super Mario Land 3 (World) ROM

Hey! Things are really cookin' on Kitchen Island! Wario's got a scheme to steal the pirates' treasure so he can have his very own castle! You help Wario get the treasure by smashing into the magic pots! Each pot changes him into Dragon Wario, or Jet Wario! With each change comes new powers!
Dozens of enemies try to stop Wario, but you have the power to beat them!

This is one of my all-time favorite GB games. You play as Wario (Mario's evil counter-part) on a journey across the landscape searching for treasures. The gameplay is similar to the two Super Mario Land games before this, but this one introduces a new cast of enemies, new power-ups, and of course, the extensive game map (with secrets you can go back and find later even). All in all, if you like mario games, or side-scrollers, or both, this game is definitely worth checking out.