"I started with Guards Guards, got hooked and decided to start at the beginning."
Me too, in the very early 90s at the point when there was only Eric and possibly Moving Pictures already published after it (I definitely remember waiting impatiently for Reaper Man to come out). If I'd started with CoM / LF I doubt I'd have perservered, because I don't read conventional fantasy (if that's not an oxymoron) and they are very much a parody of that genre so no doubt most of the satire went straight over my head, plus as others have said he was still finding his 'voice' - compare the Granny Weatherwax of ER with the later incarnation, for instance - but knowing that they evolved into the delights of Guards, Guards kept me at it. And I do love The Luggage.
Wyrd Sisters was the one that then rubber-stamped my love for TP, having studied Macbeth at A level, and ever since the Witches and the Watch have been my favourites.
My absolute favorite is Reaper Man it's completely amazing. Read it. READ IT NOW!
his other best books (IMHO) are; Small Gods Thud! The Wee Free Men Series Amazing Maurice and his Educated Rodents (The latter two being young adult). Like everyone's said, it doesn't really matter where you start, they don't have to be read in any particular order
the first one, The Colour of Magic, is very good and might be a good one to start with. I personally like the ones about the witch Granny Weatherwax very much, the first of those is Equal Rites, which is another good one. guards! guards! Is the first of a number about the Night Watch, and is quite a good one. my personal favourites, and their main characters, are; Reaper Man (Death and the Wizards) witches Abroad ( Granny Weatherwax and Co) the Last Continent (the wizards, and their housekeeper) the Fifth Elephant (the Night Watch) going Postal (a dashing conman) unseen Academicals (the Wizards and their servants)
the Discworld has changed somewhat over time, in recent novels it has become less magical, and there tend to be fewer laughs and more discussion of social issues.