ET for Atari (I don't think it deserves the label of worst game ever, but it's developers burnt and buried millions of copies of it)
interestingly enough, Raiders of the Lost Ark for Atari was made by the exact same guy as ET was.
And Superman for the N64.
Superman for the N64 was worse then ET, by a long shot.
Sure, neither game was truly based on anything in their respective licenses.
But ET existed in a time when most games didn't have a ending, and honestly, other then the "forest" area in ET, I'm more impressed by the programming and artwork that went into ET then Superman. I can think of several games I'd rather play ET over, but I can't think of a single one I'd rather play superman over.
It's that bad.
And it was released on the N64 in 1999. It had no excuse.