Terry Pratchett is trending. If you've never read his books, consider it. Even in 2020, few things could bring more joy into your world than the right Discworld book. Trust me on this.

To answer the question: Which book should I start with? There is only one universal truth nearly all Discworld fans will tell you: do NOT start with the Color of Magic, also known as the first book in the series. Absolutely under no circumstances should this be your first book.

From there, it really depends.
Guards! Guards! is the book I started with. It's the first book in the "Watch series", which is about the Ankh-Morpork city watch. This is the series that tends to be most about humanity, cities, politics, class - but starts out being about a dragon

The Witch series is fine whatever you start with really, but you should like the subject. Wyrd Sisters if you're a fan of Shakespeare. Witches Abroad if fairy tales. Lords and Ladies for a bit of a combination - the darker side of fairies in a midsummer's night's dream.

Carpe Jugulum is the best vampire story you'll ever read. Maskerade if you're a fan of opera and the Phantom of the Opera. Wee Free Men is a decent children's book starter - if you're coming off something like Harry Potter especially.

The Rincewind series seems to resonate less with people these days, and I don't really get why, since they're some of my favourite books. It's hard to imagine going wrong with at least Eric - which is short but very very funny if you're into a comedic look at summoning demons

I must also mention: Good Omens is not a Discworld book and it was half-written by Neil Gaiman, but if you like the series you're gonna love it.

One of the most popular starter books is Mort. This is a book from the Death series - in the Discworld Death is a seven-foot skeleton. The literal grim reaper. He's a decent sort and a hard worker, and Mort is his apprentice.

Also from the Death series - you really can't go wrong with Hogfather at Christmas time. And Soul Music is a good one if you have an interest in the history of popular music.

Going Postal is not one of my favourites but it is a fan favourite. It's one of the later books but it's the first of the Post Office series. Right now, it's probably the most politically relevant book.

Finally, a handful of books not part of any particular series but just set in the Discworld. Moving Pictures for fans of silver-screen cinema. The Truth if you want to feel depressed about journalism, fake news, & all of that. The Amazing Maurice for a fun look at the Pied Piper.

One of the things about Terry Pratchett is he was absolutely incredible at telling inside jokes about any subject. I've seen people not find one book funny because they just didn't really get the subject, but then pick up a book about their interest and nearly die.

I read Guards! Guards! because it had a dragon on it and I thought I knew and understood books about dragons. It was the funniest book I had ever read.
So pick up the book that appeals to a subject you already have interest in. That's the one that'll get you.

PS: I actually love Color of Magic. Didn't at first but after not reading it for years I once gave it another go and was surprised at how funny it actually is. So if you haven't read that book for a really long time... you know maybe give it a reread.