There are many bad movies. And that's a relative factor, because what's meat for one guy is poison for the other. You can have bad movies shot on handycam with little or no waste of money, or well shot, expensively funded movies blown up by poor acting, direction, and/or script.

As for the absolute worstest, that's when tonnes of money are poured into something that turns out to be messy and bad. Hollywood usually mucks up during expensive sequels that sink.

But, the absolute worstest of worst movies would be:

Valerian and the city of the thousand planets.  A poor script doomed all the hundreds of millions of dollars poured into this movie. Not all the solid acting and direction and expensive special effects could save it from certain disaster at the box office.