This is a choice between scary and funny, between the writer who has given me the creeps more than any other and the writer who has made me laugh more. Douglas Adams wrote intelligent and hilarious stories but I'm not sure that his best medium was television. The BBC adaptation of Hitchhiker's that we covered on British Invaders was good but the books and the original radio show were better. One of the books was so funny that I remember falling off my student bed from laughing so much. I could talk about Adams' work as a writer and script editor on Doctor Who but again I'm not familiar enough with that period of Who.
On the other hand Nigel Kneale was perfectly at home on television. Along with Rudolph Cartier he practically invented event television. Quatermass has had an enormous effect on British science fiction TV and his show The Year of the Sex Olympics was remarkably prescient for the future of reality television. Then there's his work on 1984, The Stone Tape and Beasts to consider. All with plenty of really creepy moments. Kneale's projects outside of TV like Halloween III were less successful but he was great on the small screen and is my favourite British Invaders writer.