Small Gods first, it’s the best of the books that aren’t sequels to something. That’s also the one I’d recommend to friends in an attempt to hook them.
After that it gets more complicated. Reading in publication order is fine, plenty of people became fans that way, and it has the advantage of starting pretty decent and, mostly, getting better as you go.
Or if you find a 40-book series daunting and want to break it into more manageable chunks, after Small Gods, you can read each of the following in any order:
- The Color of Magic/The Light Fantastic (these two have to be read in that order if you choose to read them)
- Guards, Guards
- Reaper Man
- Wyrd Sisters
If you liked Color of Magic/Light Fantastic best, (a) you’re objectively wrong, and (b) you should read the Wizards/Rincewind subseries in publication order.
If you liked Guards, Guards best, read the City Watch (and, when it shows up later, the Moist von Lipwig/Post Office) subseries in publication order. (Optional book on this path: The Truth. It’s a standalone, but characters from it keep showing up in the City Watch and Lipwig books published after it.)
If you liked Reaper Man, read the Death/Susan subseries in publication order. (This will require backtracking to read Mort before continuing.)
If you liked Wyrd Sisters, read the Witches (and, when it shows up later, the Wee Free Men/Tiffany Aching) subseries in publication order. (This will require backtracking to read Equal Rites before continuing.)
In this second method, when you get to the end of the subseries you picked, you can either stop, read the rest of the books in publication order, or pick another subseries and follow it.
Or, freestyle! Read in any order you want! Don’t let the man tell you what to do!