The only one of those consoles/systems I've ever owned was a Sega CD add-on for my Genesis. I bought it specifically to play the Lunar series in its orignal form before they came out on Playstation. I later tried out Shining Force CD, an Ecco the Dolphin game (it was $1), Popful Mail, and Vay. The first of those four was good, the next two were average, and Vay was horrible. As for the two Lunar games, the first one -- Lunar: The Silver Star -- was roughly made, but somewhat charming (it was Game Arts' first title), while the second one -- Lunar: Eternal Blue -- was AWESOME despite having horrid load times.
Since I bought the Sega CD mainly with just two games in mind, it's hard to rate the whole system, but given that I found some decent games for the Sega CD, I'm willing to bet the Sega add-ons aren't the worst on the list. (Can't speak for the 32X in that statement, though)
The only other system on the list that I've touched in my lifetime was the Virtual Boy, which I got a brief look at while visiting a friend. I don't remember much, but I know it wasn't very good -- playing games in bright red has got to have the potential to cause brain damage or seizures or something.
Anyway, I could vote for the Virtual Boy, but in my opinion, the truth is that the worst failed consoles in history are generally ones that nobody has heard of, now. After all, the worst a console can fail you is to reward you with only a dozen game titles (or less) after you've shelled out a few hundred bucks for the system. This has actually happened before, if you were unfortunate enough to be misled into buying one in the brief windows during which they were available.