I would recommend this order:
The Lord of the Rings presumes that you have read The Hobbit. (You may very well be able to get away without reading it, but there are clear back-references.)
The Silmarillion does not presume that you have read The Lord of the Rings, but would probably not be interesting to someone who is not already invested in Middle-earth.
The Great Tales trilogy are stories from The Silmarillion in more detailed forms. They require an understanding of the First Age stories to appreciate.
The Children of Húrin is a complete and detailed version of that story. Beren and Lúthien and The Fall of Gondolin are collections of the varying versions of those tales, stitched together in a directly readable order. All three of these contain significant amounts of re-printed text from The Unfinished Tales and The History of Middle-earth. These books are written to present that material in a smoother, more entertainment oriented fashion, rather than the dryer and more academic tome of the other versions.
The Unfinished Tales are exactly what it says on the tin. They contain some of Christopher Tolkien's notes about the source of the text, but are mostly readable as stories. Familiarity with The Silmarillion is essential here.
The Letters of Tolkien is a very interesting read, and could really be read almost anywhere in series. It is not, of course, a story.
You should not read The History of Middle-earth unless you are totally fanatic. It is not a series of stories, but an extended discussion of the writing of The Silmarillion, The Lord of the Rings, and some ancillary works. If you do read it, you will want to use two bookmarks, one for the primary text, and one for the copious footnotes that follow. Expect large chunks of any story you might get into reading to be removed, and replaced with a reference to The Silmarillion, The Unfinished Tales, or an earlier volume of the series.
If you make it through The History of Middle-earth, you might as well read The History of the Hobbit. Christopher Tolkien basically skipped over The Hobbit in the main history. John D. Rateliff takes over as the chronicler, doing an excellent job of examining the evolution of The Hobbit, and how it tied into the emerging legendarium. It makes a lot of references to The History of Middle-earth, so familiarity with that is necessary. It does not go into detail on the 1965 revisions to the hobbit, so it's recommended that you pair this with The Annotated Hobbit (Douglas A. Anderson.) to get the full, blow-by-blow evolution of the text.