You are correct. That is the standard answer. I entered the L'Amour novels in media res, with some of his more standard westerns, and only later read the Sackett series.
My recommendations in regard to where to start on L'Amour are colored by that as well as the fact that if someone wants to sample L'Amour and reads Sackett's Land, which takes place partly in England during the Elizabethan Age, it's not going to be very representative of his novels in general. It's very good, just quite different.
I think very highly of the Sacketts series. It is our national saga, and the Sacketts' tentacles extend throughout many of his other stories. I think that to read this series is the best introduction to who are as Americans, and that students would gain far more from it that reading any available American history book.
I was just trying to avoid someone reading Sackett's Land and thinking it was representative of most of his other books. Again is a great book, but not very representative.
The standard answer, I believe, is the “Sackett Series,” which begins with “Sackett’s Land.”