Least favourite Discworld novel?

I love most of Pratchett's Discworld series and have read most of them more than once and some of them several times. I am not fond of most of the Rincewind books (the exception being the superb Unseen Academicals, in which he has a smaller role). My least favourite of the entire series, however - the only one I wouldn't consider giving a second read - is Small Gods, which was a huge disappointment and a struggle to finish.