Best: Going Postal, but only because Night Watch was picked so often already.
Thief of Time, Lords and Ladies and Monstrous Regiment are also high on my list.
Worst: The Colour Of Magic
For me at least, it’s the juxtaposition of the pretentious Steinman/Meatloaf lyrics with the silliness of whole thing. Plus, any Blues Brothers reference will pretty much make me go to eleven.
I see your point about the plot being a bit of a rehash, but that’s a risk with any of those books (Masquerade, Moving Pictures, The Truth, etc.)
Terry rehashes themes. I suspect because he thinks he can do better. The Tiffany books are Equal Rites done much better, Soul Music has cleaner story telling and more interesting characters than Moving Pictures.
I think Thud! is one of the better books in the series, and certainly more interesting than Jingo.
Now, now. Don’t let other voters influence you! State your firm opinion and then make your comments. 
Good to see that others share my love of Night Watch and wincing displeasure of Eric. I’m also fond of Thud! and because I love golems, Feet Of Clay.
But I’ll go with a “best” for Night Watch and a “worst” for Eric.
I hate Night Watch.
My favourite is Maskerade, followed by The Truth and Last Continent.
Worst are Lords and Ladies, Night Watch, and Fifth Elephant.
Best: Wyrd Sisters - he took Macbeth - and improved on it
Favorite: Night Watch - Sam Vimes under the Lilacs.
Worst: Hogfather (I don’t really count the first three - he was obviously still figuring it all out. Otherwise, Equal Rites.)
Least Fav: Going Postal - too much perspiration, not enough inspiration.
Hmm, I’m going to distinguish between “best” and “personal favorites”.
Best: Probably Night Watch, followed closely by Small Gods and A Hatful of Sky.
Favorites: Reaper Man and Hogfather, though I really like all the Tiffany Aching books
Worst: Hmm, there a few that totally blur together in my mind: Eric, Jingo (which I have on audio and have never managed to listen to all the way through), and The Fifth Elephant.
I read Pratchett for comfort, and I’ve never been tempted to go back to those books, unlike many other Pterry books, including the Johnny and… books.
Best: Going Postal, Night Watch, Thud. In that order.
Worst: Eric, Moving Pictures, Equal Rites.
I’ll admit that I haven’t managed to get a hold of any of the witch books yet, besides equal rights. I think I’ve read all of the others, though. Except the science ones. Guess I’ve got some reading to do.
Best: Night Watch, natch.
Worst: Monstrous Regiment.
Sourcery I liked despite it being uneven. But geez, Monstrous Regiment was awful.
I think it’ll be interesting to see which books will score high on both best and worst. 
Read Lords And Ladies first, then the Tiffany books, or possibly Carpe Jugulum.
I’m not certain that I’d even classify Equal Rites as a witch book… It has Granny Weatherwax in it, but not any of the other recurring witches, and it seems to have been retconned out of the Discworld canon entirely.
Best: Night Watch or Small Gods, tossup
Worst: The Color of Magic or Eric, tossup
Best: Small Gods
Worst: Color of Magic
I completely agree with this. There’s lots in there I still like (the Alchemists, for instance), but the leads were just bleargh.
Best: Jingo, going by the fact that my paperback copy of that is in far worse shape than any other, indicating how often I have re-read it at least partially. Also up there is the not-well loved The Fifth Elephant. I think it’s the minor jokes and philosophies that stand out for me. In Jingo, the trousers of time and Vimes joke about arresting people for “loitering with intent, and loitering within tent,” and in Fifth Elephant, the dwarf king’s story about how even if every replaceable item is replaced in an heirloom, it’s still that heirloom.
Worst: a couple of them went past just like that, and I don’t remember a thing about them. The Colour of Magic is definitely up there, but then so is Moving Pictures and Soul Music. But for very worst I would have to pick Monstrous Regiment. A whole new cast of characters, one more forgettable than the next, and then it turns out
They’re all women!
If Terry really expected anyone to swallow that sorry excuse for a plot, sorry – but his next ones were fabulous again, so I don’t complain too much.
What I think are best and worst don’t match up to my favourites, *Night Watch *is a great read but a bit flipping grim, same goes for a lot of Small Gods and Thud!.
My choices
Best: Guards! Guards! and Lords and Ladies.
Worst: Of the “proper” Discworld books I found Moving Pictures clunky.
Best: Night watch and about 20 others
Worst: Eric (and Monstrous regiment and Going postal),
What day of the week is it?
Best: Night Watch or Thud!
Worst: The Colour of Magic, probably
Most enjoyable experience was the audiobook version of Wee Free Men. The narrator did a great job of creating distinct voices for each of the characters (instead of one generic Scots accent), and the Pictsies are even funnier when you hear them than when you read them. The vocal repetition of “Not-as-big-as-Medium-sized-Jock-but-bigger-than-Wee-Jock Jock,” just made me smile every time I heard it, and the narrator skips through those names so lightly that I wonder how many takes were required. It’s even a bit funnier now that I find that “Jock” is, or was, English slang for a Scot.