As a huge fan of everything Final Fantasy, it really pains me to say it, but... FFXIV.
Actually, if we're talking all time, when I was but a child and money was extremely hard to come by, buying a bad game was a catastrophe of epic proportions. This was at a time when the Internet did not exist and researching the quality of games was much more difficult, with only dubious imported gaming magazines serving as any sort of guideline.
So, with that in mind, I'd have to say... maybe Road Rash 3 on the Mega Drive. It's actually a really good game, but I already had Road Rash 2, and it just didn't seem to offer enough that was different to justify the relatively vast sum I had paid for it.