The problem with starting is that the very early books aren't that well written. They're funny and imaginative and great stories, but the actual text is a bit difficult. I found this out when reading them out loud to my oldest who was a bit slow to reading (fine now).

The recommendation I've had and am happy to pass along is to begin with Guards! Guards!, which is the first novel where it really becomes evident what Pratchett has to say now he's gotten all the juvenilia out of the way and set to work in earnest.

The series is certainly worth the trouble, though possibly not if you approach it with the expectation of being graded.

I started in publication order, and while I agree the writing is not as refined in Colour of Magic, it does at least lay the groundwork for some of the overarching themes of the series.

I actually thought Light Fantastic did a pretty good job wrapping up Colour of Magic, though I would probably tell people to start with Mort.