Reaper Man is an excellent standalone story featuring Death (tall, bony, robe and scythe...) as the protagonist. Wyrd Sisters is the first of the Witches of Lancre series (discounting Equal Rites, wherein Granny Weatherwax appears but is not depicted quite as her fully formed character yet), and Moving Pictures is the first of the series dealing with various phenomena which attempt to modernize the world.
Small Gods is also broadly a standalone story which mercilessly skewers the rise of Christianity from its humble desert beginnings up to the largess and insanity of the Inquisition, except with the key difference of the god in question being very much real, even if none of his ostensible followers except one actually believe in him anymore — despite the rest of them going around behaving very badly in his name.
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