Hmmm. There are many "sub" series of Discworld books, like the Rincewind tales, Witches, etc. Pls refer to following image:

But it's just a rough guide, my 1st Discworld books were Small Gods & Hogfather. Based on above, I suggest starting with the first 3 of either the City Watch books, Witches and/or Death books.

Try Good Omens, which was written by both of them. If you like it, you'll definitely like at least 75% Pratchett's stuff.

Pretty much anywhere in the reading order diagram I posted earlier in the thread. Heck, you don't even have to follow that thing, my first Discworld books were Small Gods & Hogfather.

But if you're a Gaiman fan, I suggest giving the Witches or Death sequences a try first, they have more overlap..

Just finished I Shall Wear Midnight, & read Unseen Academicals some time ago...

UA is actually very good, but is built on a lot of previous Discworld stories, despite having mostly new main characters.

ISWM, the "Young Adult" novel, is - like all of pTerry's YA books, darker in tone than the normal Discworld books. It's at best average as the story just doesn't seem to go anywhere.

Hope it's good... & wonder if Lewton from the Discworld Noir game will show up