Since it has not been mentioned yet, the Memorex/Tandy VIS is certainly a contender for the list, to say the least.

Regarding above posts, there should be a distinction made between raw sales numbers and the cost of those sales. For example, the Neo Geo AES did not sell in large numbers, but it was never unprofitable for the company making it, as most units made DID sell. This is contrast to systems like the VIS, which barely sold more than a small percentage of the units produced and ended up costing its makers millions upon millions.