I followed the prescribed unofficial Reading Order graphic when I started the series (only 10ish books in), and I've jumped around between all the different protagonists. Color of Magic, Mort, Guards Guards, and the Equal Rites do well enough to show you what the Disc is all about, the absurdity of normalcy, and some of the crazy antics people get into. I think it may be more a matter of what "type" of fiction are you looking for - a constantly terrified Rincewind in a world of magic, a new to the job Mort where no one believes HE is Death, a tired Sam Vimes trying to understand a world that is changing faster than he can keep up, or the witches the reject all the social rules of the land. Really the only one I wouldn't recommend first is Small Gods, but mostly because its very self contained to the Klatchian region while the others are on another continent.