I'm a big proponent of publication order most of the time, for most series, and I do love the first Rhincewind books, but I keep seeing people saying they have trouble with them as an introduction to Discworld.
I have trouble understanding this because I love the first Discworld books. However, I think they rely on a lot of referential humor that you just don't get if you're not familiar with the source material.
You have to have read Fafhrd and the Gray Mouser, Conan, Lord Dunsany, Vance's Dying Earth, McCaffrey's Pern, and etc.
It helps to be familiar with UK culture of the time, various folk and fairy tales, the kinds of things an English schoolboy would know about Ancient Greece and Egypt, and a potpourri of other things to appreciate a lot of the jokes.
So unfortunately I can't provide a positive answer to the question of which books to start with but do have to reluctantly agree that the first Rhincewind books are not a good choice for a modern reader.