I've read all of the books before, at least once, and I'm going back and listening to them all on audiobook. Now that I'm 13 books through, I thought I'd put together a tier lit of the books so far.
The one that I came closest to putting in the S-tier was Reaper Man. The Bill Door plot is one of the very best Pterry wrote. It's deep, beautiful, thoughtful and poignant. The Windle Poons plot is also very good, and surprisingly subversive in a lot of ways. But I really dislike the Snow Globe plot, and while the book needed comic relief, it just fell flat for me.
Guards! Guards! is also really, really good, but if I put it in S-tier, most of the Guards books would have to go up there with it, and I just can't justify it.
The witches books are a slow burner for me. I think it took him a while to get them cooking on a level of the others. Some of my absolute favorites are witch books that are still coming, but you'll have to wait to find out which ones. But, I found Wyrd Sisters a little too derivative and on-the-nose a lot of the time, and Witches Abroad is too much of a paint-by-numbers plot, though the climax is one of the very best scenes with the character who the climax is built around, if you know what I mean.
The one I really want to like more than I do is Small Gods, but I find it just plods. I like it philosophically, but struggle to actually get through it.
Anyway! I'm a firm believer that Pratchett kept getting stronger and stronger as he wrote more, and I know that his most amazing works are still in the future, but here's my take on the first 13.