I'm voting for Wario Land III because, as TYP demonstrated, it is held in the very highest regard for portable games of the GB/GBC era. An interesting side note is that Wario Land III was developed by Nintendo R&D1, the creators of Metroid and also the developers of Super Metroid, Fusion and Zero Mission. Metroid II, however, was developed by Intelligent Systems, best known for the Famicom/Advance Wars games and Fire Emblem. The two studios have also split the development duties on the WarioWare games, with Intelligent Systems being responsible for Touched and the recent DIY, I believe.