Same as the regular consoles, only the portable versions. Going right back to the dawn of time here for some, with the Sega Game Gear and the Nintendo Game Boy. As with all these golden-oldie console debates, it's basically whether you preferred Nintendo or Sega games, although the main rivals here are perhaps not Mario and Sonic, but the legendary Tetris and the not-so-legendary Columns. Enjoy voting!
The Game Boy covers all incarnations of the console, from Game Boy, to Game Boy Pocket, to Game Boy Color. The Game Boy Advance and Game Boy Advance SP will feature in the next (and current) generation of handheld consoles.