opinion is also split on where to start, since it is generally agreed that the early books weren't the best, and yet the recurring characters do develop over time. I started with Feet of Clay, which took me like 3 attempts before i finally "got it". I now often recommend Small Gods to people, because it is a self contained book with only cameos from recurring characters, and it has really nice commentary on religion, reflexivity, and the role of belief in our actions and realities. I also particularly recommend the Night Watch series, just because Vimes is pretty much Terry's self insert, and as a business minded person I also loved the later Moist series (eg Going Postal).
GNU Terry Pratchett. may your timeless stories be endlessly told and retold.