Since, ET for the Atari 2600 is widely held to be the worst video game ever, I would have to lean towards Pac Man.
Levels 5 - infinity of Atari 2600 Pac Man could all be beaten by the exact same pattern, but at least you got five different levels at the start. ET, on the other hand had exactly the same adventure throughout (if you call falling into pits and levitating yourself back out at a rate roughly equivalent to the walking pace of a turtle on downers an adventure).
ET was so bad that Atari was forced to dump 5 million cartridges in a landfill in New Mexico. Then they crushed the cartridges with a steam roller, and poured cement over the whole ugly mess. To this day that entire section of the desert is still uninhabitable due to the toxic effects of ET on higher brain function (I made this part up).
