I would say the Sega Nomad battery pack, unless you like killing full packs of brand new batteries daily. It's better to take the nomad places where you can plug it in the wall, like motels, friend's houses, your house, backyard w/ extension cord etc.
Unless you mod it and replace the backlight with a more energy efficient one.
Here's another - The Commodore VIC-20 modem. Who's going to connect their phone through a VIC-20 these days?