In my opinion he started off with one style 'Colour of Magic' and 'Light Fantastic', and then spent the next few books tweaking things*, and changing his writing styles and techniques. In my opinion Sourcery and Pyramids were some of his weaker books because of this. I still enjoyed them, but didn't feel much desire to re-read them. Admittedly they've been in a box for the last 15 years, and I've only just dug them out after several moves, and some time in storage - perhaps they're due another read?
Anyway, I think he'd really got himself going again with Wyrd Sisters. Which I very much like (and have re-read). And might be a good starting point. I love the way he can be doing a multi-page literary parody, sometimes within a book that is already a literary parody (Wyrd Sisters / Macbeth for example) - and then he'll put in a proper groan-inducing pun and suddenly there'll be a knob gag, or he'll drop in a stupid footnote for the hell of it. His writing style plays on so many different levels that you can appreciate the books in different ways, depending on your mood, and that makes them worth re-reading too. So Wyrd Sisters then Witches Abroad (where I loved the short Hemingway parody) then hooked.
Or you could go for for Guards! Guards!, and read along with the Watch, as someone else also suggested. There's also the Death novels. But I didn't think Mort was one of his strongest books, so I wouldn't start with it, just like I think Equal Rites (the first witches one) isn't even in the same style as his later stuff. Everyone likes different things though. And I do fear recommending books / music that I love to people, because so much of it is personal. And then when they don't like it, they've stabbed me in the back. The bastards!
You've got a whole load of books to look forward to. Assuming you liked the first couple you read. Happy days. There's a bunch of kids/young adult books in the Discworld series as well, but I just ignored that and read them anyway. I really enjoyed the Tiffany Aching books (Wee Free Men, Hat Full of Sky and Wintersmith). More witches.
He then did a bunch of other stuff. I remember not particularly liking Strata and Dark Side of the Sun - but that's all I remember, I read those sometime in the early 90s I think. There was Truckers, Diggers and something else, which I've never read, and I seem to recall the name 'The Carpet People'. All kids books I think.
Then finally, Good Omens (with Neil Gaiman). Radio 4's recent adaptation was excellent. The book is even better. Hopefully, happy reading to you. Oh and the ones with Stephen Baxter, that I haven't read, the first one's on my bedside book pile.
* I just had a major fat-finger issue and typed twerking1 (should I have left that in?).
1 I put this comment in brackets, then realised that in honour of the Master himself, it should obviously be done as a footnote2. I also particuarly liked his footnotes inside footnotes gags.
2I should probably have stopped after 2...