Worst vs. Most disappointing?
I think that one should separate the concept of 'worst' from the concept of 'most disappointing'. For example Avatar was not the worst film ever made, but is a candidate for 'most disappointing' because it could easily have been so much better. The cinematography and special effects were really pretty good, but the triteness of the script was enough to really spoil the film.
The Phantom Menace and The Matrix Reloaded were deeply mediocre films that followed a much better predecessor, but 'mediocre' is probably a better definition than 'crap' (OK Jar Jar Binks was actually crap and some of the antagonists in TMR were pretty rubbish as well). This makes them contenders for 'most disappointing', but not necessarily 'worst'.
I haven't seen Battlefield Earth, although I'm told it is really pretty terrible. Of the ones I've seen, I think Highlander 2 is comfortably the worst on that list in terms of absolute production values. It really was poo - crap script, crap production, crap acting, crap premise, tenuous and contrived link to the first that completely bastardised several the major tropes. Michael Bay would be proud.