The Colour of Magic is a reasonable place to start, but it doesn’t really show the Discworld at its best. It’s very specifically a parody of some popular SF/Fantasy series of the late 1970s and early '80s (notably Anne McCaffrey’s Pern and, more fundamentally, Fritz Leiber’s Fafhrd and the Grey Mouser), and the jokes can fall flat if you don’t know the books that he’s parodying.
The series has moved to more serious territory since then, and has at the same time actually gotten funnier. (Pratchett’s own jokes are much funnier than his parodies of other people’s silly ideas.) Small Gods is probably the darkest and least funny of the series, so it also isn’t a great starting point. Since that book, I think he’s balanced the light and dark elements very well to tell serious moral tales that are also hilariously funny.
The Rincewind books are also fundamentally sillier and more loosely-plotted than the other books (since they consist in large part of:
A) Rincewind confronts danger
B) Rincewind runs away from danger
C) Danger chases Rincewind until it gets out of breath).
Not that they don’t work as humorous novels, but the other series have more complex and involving plots.
I’d suggest starting with either Wyrd Sisters (especially if you’re of a Shakespearean bent), Mort, Pyramids or Guards! Guards!. Those all begin sub-series or (in the case of Pyramids) stand alone. They all also show the Discworld’s strengths well, which I don’t think is quite the case for The Colour of Magic.
Strata shows Pratchett playing with the ideas that will eventually become the Discworld (and, incidentally, parodying Larry Niven’s Ringworld), so it’s much more interesting to read after you’re familiar with the Discworld. The Colour of Magic can cause similar problems; Pratchett was still working out how the Discworld worked while he was writing it, so it doesn’t entirely have the same tone and feel as later books in the series. The Light Fantastic is a direct sequel to The Colour of Magic, and is a little more like the later novels, but I think he didn’t really crystallize what the books were about until Equal Rites, the third novel.
So I’d argue against the crowd: The Colour of Magic is fun, but not really representative of the series as it developed, and so it isn’t the best introduction to Pratchett.
Good Omens, on the other hand, is another good introduction to Pratchett, especially to a person coming from comic-dom or otherwise already familiar with the co-author, Neil Gaiman. But it has nothing to do with Discworld, which might make it a bit beside the point if you’re looking for a big, long-running series to dive into.