Just because you didn’t like it doesn’t make it the worst. You need something truly horrible, with horrible script, acting and production values.
Manos: Hands of Fate is a good example. That movie has absolutely no redeeming value. It’s hard to watch even on B-movie night, whereas Plan 9 is bad but fun to watch.
But low budget isn’t the sole reason. The high-budget Battlefield Earth also had no redeeming value, making it among the worst films ever. The plot, script, acting, costumes, makeup, sets, cinematography, all horrible. I even liked the book and like John Travolta as an actor, so it’s not an anti-Scientology thing.
All very well, but it does for me. And frankly, my opinion is the only one that counts for me. Sure, I can recognize the production values, and the artistic values, the good acting, whatever. But I'd rather watch "Attack of the Killer Tomatoes" a dozen times than sit through "Seven" again.
And after all, it's just an opinion. The only thing that make it important to me is that it's mine. I hated "The Wild Bunch", too, for that matter. I guess it depends on what your aesthetic is, but mine hates both those movies, and wishes I had my money and my time back.