ET being the worst is mostly kind of a meme. It's a bad game, but its badness is far overstated. It's just a bandwagon that people jump on because it's funny or because the narrative is compelling. That it's the game that almost killed an entire industry, or the bit about all of the millions of carts dumped in a landfill (which is really more about bad forecasting by whoever chose how many to make) -- those are narratively satisfying, and it's fun to dunk on bad things, so the story spreads.

I'm pretty confident that there are worse Atari 2600 games, without even spreading the search farther. They just didn't meme as hard.