I’m not including a vote for the stand-alones, just the main series in the books.
What is your favorite? Why?
The city watch, which was consistently good and funny. The Shakespeare parodies in The Witches were clever, too.
City Watch for me too(I’m on Night Watch right now).
I admire Carrot and Vimes, two fictional characters I actually find myself looking up to. Not many series I can say that about.
I’m tempted to cheat and call Tiffany Aching a subset of the Witches. But then, if I were going to cheat, I’d just call everything except for The Wee Free Men a DEATH book, and vote for those. So I think I have to go with Tiffany.
Witches is probably in second place, but that’s on overall quality: Some of my favorites are DEATH books, but some of those are pretty lackluster, too. And while I love Vimes and Carrot (and Detritus, Cheery, and Angua) as characters, some of the stories they’re in aren’t too great.
Moist von Lipvig, of course, has two great books and one stinker.
Tiffany, but it’s not like it pulls out ahead by a mile. My favorite book probably is Witches Abroad and my favorite character might be Vimes or Death or Granny. There’s a lot to be said for all of them, even the wizards.
I haven’t read any Tiffany Aching yet, and only some of the Witches books, but for me, it’s still the Watch books by a mile. I love Vimes. 
Wizards (and one wizzard) for me. I love all the book series, but will always have a soft spot for Rincewind.
I choose to include the* Science of the Discworld* books in there.
Although, of all Pratchett, if the Johnny Maxwell books were included, they’d get my vote instead.
I like them all for different reasons. And The Witches dovetails into Tiffany Aching so closely I’d say they’re the same series.
Death’s stories get a bit repetitious. I have great love for Rincewind, but I recognise his stories lack bite. I also like the City Watch but unlike most people I really don’t like Vimes as a character.
I chose Tiffany Aching’s series, they hit all the right notes consistently for me.
Witches Abroad is just quintessential Prachett. Taking fairytales and turn them inside out.
Make sure Cinderella doesn’t marry the Prince.
So far, I have only read from four of those, and then only one or two each. But of those I like Going Postal the most. So my vote’s for Moist.
Very close between the witches and the Watch, but when I found out that my cousin had never read any Discworld books (and he is the kind of person who would love them) I recommended that he start with Guards, Guards.
I voted for Moist, but I could just as happily voted for the witches or for the watch. Loved them all.
Oh, and what about Equal Rites and Lords and Ladies? Are we putting those in “witch” or “wizard”?
It’s really up to you since we’re looking at overall series. I would look at both series with those novels included and see which series is superior.