Ok, i got this idea from a similar thread at FSM about Goldsmith's worst film (not film score).
According to imdb ratings (see here at his composer credits list):
Daddy-O is the worst film Williams has scored (with a rating of 2.30, followed by Heartbeeps with a rating of 4.25)
Schindler's List is the best (with a rating of 8.90, followed by Star Wars and Empire Strikes back with a rating of 8.80)
Just thought that it would be interesting to point out.