I don't know how well it comes across, but I've tried to be fair to these early Newbery winners. While not glossing over their flaws, I've tried to show how each one fits into the emerging story of American children's literature, note the things that each one does well, and place the books in the context of their time.
My friends, all of that fails me when I come to Daniel Boone, James Daugherty's 1940 winner. It's a self-satisfied hymn to racism and Manifest Destiny, accompanied by hideously ugly (and somehow even more racist) artwork by the author. The pacing is terrible, and the prose confuses overuse of adjectives for inspiring writing. It doesn't even work very well as a biography -- it doesn't have a timeline, assumes far too much background knowledge on the part of its readers, and sometimes fails to even refer to its many poorly described characters by their full names.
No libraries in my consortium own Daniel Boone; I had to use the statewide interlibrary loan system to even find a copy to read. It's completely out of print, a distinction that might make it unique in the Newbery canon. Even the most deeply problematic Newbery winners stay in print -- Shen of the Sea, Hitty: Her First Hundred Years, The Matchlock Gun, all are still easily available, straight from the presses. But if, for whatever reason, you want a copy of Daniel Boone, you're going to have to find it on the secondhand market, at prices that are often north of $100.
Part of the problem is that the racism (mostly directed at Native Americans, but with jabs at African Americans as well) so thoroughly permeates the book that it would be impossible to produce an edited version, as was done for The Voyages of Doctor Doolittle and Rabbit Hill, and have more than a pamphlet left. I'm reminded of Roger Ebert, who, when speaking about software designed to remove offensive passages from DVDs, opined, "Theoretically there could be a version of Fight Club suitable for grade-schoolers, although it would be very short."
Daniel Boone himself is a complicated historical figure, and certainly one about whom a fascinating biography could be written. It's hard to get a sense of the man from Daniel Boone, however -- his personality is flattened into a caricature of a frontiersman. I don't feel like I know him much better after finishing the book than I did before I began.
James Daugherty was well-regarded in his day, both as an author and as an illustrator -- he picked up two Caldecott Honors as well, for Andy and the Lion (1939) and Gillespie and the Guards (1957). I haven't read either of those, though I do note that Andy and the Lion at least is still in print. I can say that, at this remove, Daugherty is not a major figure in the history of children's literature. If Daniel Boone is representative of his work, it's easy for me to understand why.
The only remaining question I have is this: is Daniel Boone the worst Newbery winner ever? I think it depends on what criteria you want to use. The other real contender, in my personal view, is Smoky, the Cowhorse. Smoky is probably three times the length of Daniel Boone, and might be the single least interesting book I've ever read; from a purely technical perspective, I'd argue that it's worse than Daniel Boone. But if there's a more nauseatingly racist book among the Newberys, I certainly can't tell you what it might be; I'm unable to come up with a good ethical defense of Daniel Boone, and from that perspective, it might represent the actual bottom of the Newbery barrel.