Wow...it's hard to pick a best, and I don't even want to try for a worst.... I haven't read anything lately--the libraries here suck, so they've got a limited selection of his work. I drove 'cross country last year, though, and stopped in libraries along the way, to find books to read...in doing so, I finished most of Strata--really want to finish that one. And Wintersmith--also a fairly good book. But now I think about it...neither of them are Discworld.
Anyway...the first of the series, that I read, was Pyramids--I was wandering through the library, looking for something to read, preferably something with lots of parts, to keep me going for a while. Then I saw a shelf completely full of stuff by the same author, (this is where I used to live, and they DID have a nice selection. =P). I picked one out, (Pyramids), and tried to find a guide for where the series started. There didn't appear to be one, so I just went ahead and got that one. Since then, it's always been one of my favourites. After that...Lords and Ladies I liked, because it's a parody of Shakespeare's, "A Mid-Summer Night's Dream", which I was in a short time before. Thief of Time is good. As is The Truth. I also like the Rincewind series, especially Eric, though that might be, in part, because I had to hunt down a copy to read it, so I was bound and determined to enjoy it.... But mostly, I think, because I'd just read Goethe's "Faust". I generally like most of the books with Lu Tze. He's awesome, him and all the History Monks....
I've never been quite as much a fan of the Witches series, but that may be because it was the last I started on. After I picked a few random ones, which turned out to be pretty much all disconnected singles, I told my sister I was reading his books, and it turns out she was a huge fan, and got me a copy of the flowchart on L-Space. That was nice. From there I read the Rincewind, then kind of went on the Death and Watch series' at the same time, so I like them about equal. My sister told me to read the Witches last, because they're a bit more complex, and take some thought...and since I was, like, 14 at the time, I wasn't much into thinking. =P
But now...a question. I remember part of a joke, from one of the books...one with the History Monks. Someone says something along the lines of..."If monastaries have monks, what do llamasaries have?" That was several years ago...for quite some time, I've been REALLY wondering which book it's in...can anyone help me out?