Hi marky77. I am an avid Discworld fan, and as such, I believe that all books in the series are very funny, & well worth the read. If pushed into a corner, I think that "Witches abroad" is brilliant, the three of them wreaking havoc across the disc. "Small gods" is also very, very funny (and thought provoking), all of the remainder are highly amusing, but the first two ""The colour of magic", & "The light fantastic" are probably best avoided untill hooked. Enjoy.
You may have noticed they go in series - Death and his family, the witches of Lancre, the guards in Ankh Morpork, the wizards of the Unseen University and what you might call miscellaneous. I would suggest you try one of each and see if you get hooked. I would recommend "Mort", "Witches abroad", "Guards, Guards" (my favourite is actually "Nightwatch" but it is a bit darker, more homage than satyre), "Eric" and perhaps "Small Gods" or "Going Postal" in the miscellaneous section. Tiffany Aching has become a spin-off of the witches series and it is also very good (try "Wee Free Men"), and the "Science of Discworld" three books to date are surprisingly readable, considering they get everywhere from the Big Bang to quantum mechanics or the theory of evolution.
Marky I think you've found Terry's next plot. Discoworld sounds good seeing as music 'n' dance media are enjoying a revival with the remake of Hairspray etc. Each book has its own qualities, take your time and read them all and see which characters you most like.