The Sega Game Gear. Released in 1990 to compete with Nintendo's Game Boy, on paper it was a no-contest winner: colour graphics on a backlit display, a more common-place processor, twice the VRAM, compatibility (via an accessory) with the entire existing library of Master System titles (including an adaptor for 2-player games), and even an analogue TV tuner accessory made it seem all but certain Nintendo would have to react or fall by the wayside.
Sadly, something went wrong for Sega. Nintendo didn't add colour (or a backlight in the global market) until 1998, by which time the Game Gear had already been dead for a year. While the Game Gear had comparatively short battery life (especially considering it took more batteries) and some questionable titles, it was ultimately Sega's lack of focus on the mobile market which sealed its fate.
