It's gotta be the Wii U + 3DS in my view.
The 3DS wasn't a total loss, but it was a massive decline from the DS selling less than half as much lifetime, and it required an emergency price cut just to get off the grounded after a bungled launch. Wii U was just a disaster, one of the lowest selling major consoles ever, and sold just over a tenth of the Wii.
The Gamecube was a flop as well, but in that case the GBA was there to bail them out.