Virtual Boy.
Had they released it as effectively the same thing but with an 8-bit color palette (heck, I think even 4-bit would've sufficed), I think it would have done vastly better than it did.
It also had a bad form-factor - if it had headstraps like modern VR systems, it would've been more palatable.
Both of these issues probably would've been resolved with a longer dev period, but it is what it is.
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