Nomination: Philips DCC
Philips tried to redefine the C-cassettes as a digital format: Recording compressed audio digitally (some for of MPEG2-compression I think) on tape cartridges the same size as classic C-cassettes. This was supposed to be also backward-compatible: DCC recorders could play back (but not record) analogue cassettes, even though physically the cassettes looked quite different. The DCC cassette had a sliding tape protector, and was inserted only in one way, no flipping by the user needed. (It still had A and B sides, but switching between these was handled by the deck).
The sound quality was actually not bad, I owned a deck (probably still somewhere in the cellar) when they were in firesale mode. CD quality, as far as my ears could tell. But the format was harder to use than analogue cassettes. You could not just throw them in, and start recording, some formatting was needed. Also any dirt in the recording head killed it, and one way to get the head dirty quickly was to use the advertised compatibility feature and play back analogue tapes...
Also it had the problems of tape in seeking to a desired track. It could seek automatically, but it took time. The rival Sony MiniDisk did not have this problem, so it won, sort of.
Both formats were of course finally obsoleted by MP3 files on the Internet, and MP3 players.