So I was a bit apprehensive about the Discworld books, even as they approached massive popularity with more and more fans, including several of my friends. In the USA, Pratchett has fully caught up with Douglas Adams as the face of modern British fantastical drollery. Finally I gave in and started buying his books as they appeared at used bookstores.
My first Discworld book was Jingo, which probably was not the best choice for a beginner, as it focuses on a group of characters it assumes you're already familiar with (Sam Vimes and his cohort).
My second Discworld book was Monstrous Regiment, which was published later but is probably a better choice for a beginner. It's firmly set within the Discworld universe but it's more of a standalone, as it introduces a brand-new group of protagonists.
Both of these books were written after Pratchett had been writing Discworld books for a while. He'd settled into a comfortable groove, in which he parodies the tropes of several different genres within a single novel.
That said, I think it's a mistake to consider the mature Discworld novels to be parodies first and foremost. To me, a parody isn't just any humorous work that can be slotted comfortably into a particular genre. A parody is a humorous work that derives most of its humor from mocking the tropes of a particular genre. But most of the humor of the later Discworld books is character-based.
So, for example, while Monstrous Regiment does indeed parody (verb) many military fiction tropes (and thoroughly deconstructs the hoary 'brave young girl disguises herself as a man so she can enlist in the army' trope), you can't just label the novel as a whole a parody (noun). That's so limiting.
Anyway. My third Discworld book was the very first Discworld book: The Colour of Magic. As one can expect from the very first installment of a long-running series, The Colour of Magic features loads and loads of what TVTropes calls 'Early Installment Weirdness'. The novel is divided into chapters (each subsequent Discworld book would be one continuous undivided story), the story is much more episodic and disjointed than later books, stupid jokes and cheap humor are much more in evidence, and everything's a parody! Every segment of the episodic plot is a parody of a different sub-genre of fantasy.
That said, The Colour of Magic isn't a bad book. It's well-written, and Rincewind and Twoflower are both very nicely done comic characters. But it's not typical Discworld.
Which brings me to The Light Fantastic, #2 in the Discworld series. The Light Fantastic has much in common with its predecessor. Later Discworld novels would each be self-contained stories set in a well-established universe with recurring characters, but The Light Fantastic is a direct sequel to The Colour of Magic, beginning seconds after the first book ended with Rincewind and Twoflower launched off the edge of the world. The two books are one episodic novel split in half.
It's interesting to view the book as a snapshot of Discworld evolution. #2 continues the plot from #1, and like #1 it contains its share of painful jokes. (Upon hearing that the civil unrest has resulted in a mob ransacking the city's music stores, Rincewind shakes his head. 'Luters', he mutters.) It's also not all that great with its female characters; every woman in the book is a parody of a particular fantasy trope, and gets little character development beyond that. (Pratchett would get much, much better at writing women in his later books -- see Monstrous Regiment as proof of that.)
And while there's plenty of character-based humor, in The Light Fantastic it seems like Pratchett is more likely to, say, describe some aspect of Twoflower's personality in a humorous way, rather than have Twoflower actually do something humorous that fits his personality.
But there are also signs in #2 that Pratchett is settling into the groove he would ride so successfully (and profitably) in the years ahead. The Light Fantastic comes much closer to giving its readers a single cohesive story than its predecessor. It's not divided into chapters. And people who are far more serious Pratchett fans than I am claim that many of the long-running elements of the Discworld series get their start in The Light Fantastic.
I'll take their word for it -- after all, so far I've only read four novels out of -- let me check Wikipedia -- wow, 39 in total so far. Ever since I read Jingo in 2008, I've read the Discworld books at a rate of one a year. I find Pratchett to be a pleasant enough read, but I'm not sure if I'll increase my Discworld novel consumption speed.