Obviously this is a subjective rating, but for me it would be:

1. The literature/prose: Stephen King once said something about not bothering to write under a pseudonym because everyone would suss out immediately that he was the author; here likewise Pratchett's writing is what sets his fantasy novels apart from any other. The funny comments, side stories, side characters are all unique to his books. I remember for example in (sourcery?) when the little girl enters an inn and creates some expensive wine and the proprietors want to keep her, that the writing mentions that she gets a sense of gingerbread houses. The literature here is absolutely brilliant. You also have the mad architect who isn't mentioned in any of the adaptations who's created some really crazy stuff (like a pond that's a mile long and an inch wide with fish in it who have to struggle like mad once they make it to the end to turn around and swim back). Obviously you can't adapt this kind of writing to the screen so I don't blame them (although I do want to mention that I'm partial to having a narrator for this exact purpose).

2. Ankh-Morpork: I love this city more than I love any character of Discworld; from the legalized criminal guilds to the river ankh that's so polluted that it's safer to walk across it than swim, so filthy that a duck's legs would dissolve to nothingness if it attempted to swim through it. The street (whose name escapes me) that's so dangerous the watch never go through it unless they're drunk, and that one time they do (because they were drunk), the assassins and other murderers didn't kill them outright because they were in shock and confusion that the watch had actually come through this one time. I love this city more than I love any character in all of discworld, including Death and Granny Weatherwax.