I can see both sides of the argument (for me it's got to be Delia Derbyshire), but nobody can say that what she did was just "tinkering to make a new sound", what she did was evolutionary, electronic music today wouldn't be the same without the original Doctor Who theme. And if you want to see how much work went into the original Doctor Who theme, just watch the documentary in the "Doctor Who: Beginning" boxset. But at the same time, John Williams is a great composer, but he just wouldn't cut it if he tried to do the Doctor Who theme. He may seem more sophisticated with his music than Delia Derbyshire was, but she was fantastic at what she did, creating electronic music that could scare the pants of you.
They are both great composers so there's nothing to really complain about, their music is for people to enjoy, not to fight over.