I'd say films like Independence Day (a well-done bit of jolly romping hokum that didn't take itself too seriously) and Burton's Alice (disappointing overall but with some good bits) can't really be described as the Worst Movie Ever.

But something like Prince Valiant (the 1997 version) genuinely is a candidate... no doubt there are 100s of worse films I will never see, but this was incompetent and unfunny and woodenly-acted and with some useless effects... read reviewer James Hitchcock at the IMDb link, who is pleasingly 1) disgusted and 2) from Tunbridge Wells.

And it is not as though it is full of unknown actors - people like Edward Fox and Thomas Kretschmann and Joanna Lumley are in it, and Katherine Heigl also; here she was 18 and very wooden, though I gather she got better.