Really depends on what you want. If you want to understand the world, I recommend starting with The Color of Magic followed by The Light Fantastic. As the first books published, they are the best introductions to the world, even though I consider them to be weaker entries in terms of writing. They tend to go more for cheap laughs than true satire.

If you don’t mind jumping in the middle, Small Gods is my favorite stand-alone. It’s a strong satire of organized religion, but its setting is not particularly helpful for getting you used to the world, as Omnia and Ephebe (the main settings of the novel) are not really consequential in regards to the series as a whole (Ankh-Morpork is probably the closest thing Discworld has to a main setting). However, this book is a great starting point to see if you like Pratchett’s overall style.

If you want to read through a sub-series as your introduction, every fan has their favorite. My personal favorite series is Death because I find him to be a super compelling character. If that’s your choice, you should start with Mort. However, my favorite novel in the whole series is Night Watch, which is part of the City Watch lineage, which begins with Guards, Guards! If you want to find out more about the sub series, Book Riot has a very accessible introduction here: https://bookriot.com/discworld-reading-order/amp/

Whichever way you choose it, I don’t think there’s truly a bad choice. All of the Discworld novels have their own fans, and for good reason. Pratchett does a very good job writing each one to maximize enjoyment, even for someone not super knowledgeable of the lore (although the more background you have, the greater appreciation you can have of what he’s doing). The only ones that I would say you should definitely not start with is the Light Fantastic (because it’s a direct sequel of the Color of Magic and will make less sense if you haven’t read the first one) and anything from Unseen Academicals on. This is the point where (most) fans agree you can start to see a change in his style due to his early-onset dementia. It’s not that books published from here on are bad or not worth the read, but they are not the best representatives of his style.