There are so many interesting characters in all of the writing of Terry.
I find it hard to pick just one...I love the witches but I also am very fond of Rincewind.
Ooooh, a tough question! Granny Weatherwax and Death (later versus earlier) are my two favorite. I will say that Making Money made me start to like Vetrinari some more. I also really like the books that take place on Uberwald.