I can't pick one - too many of them are good, and I don't work on the basis of "favourite" to a great deal, because it's often subject to change, no matter the subject matter.
However...
Honourable mentions go to:
Vimes, from Feet Of Clay onwards at any rate. Jingo and Night Watch are the two where he really comes into his own.
I'd take the Death of Rats over Death any day. Death is amusing, I'll grant, but there's just something about seeing the word SQUEAK.
The Dibblers in all their many nationalised formats.
Lu-Tze. If only for Rule One. "Do not act incautiously when facing small unarmed wrinkly old men!"
Om, in Small Gods. The way his attitude changes over the course of the book is a joy to see, and it's a superb take on religion.
The wizards of Unseen University, but only when you have some of them in a group. A personal preference for the Senior Wrangler, because I can often be pedantic.
The Duck Man. "What duck?"
There's so many to choose from, though, that I can't say I have a favourite.