Honestly? Awful. Halo novels are the definition of potboiler, they've got less literary merit than John Green. It's blatantly obvious they were written purely to cash in on the brand name. IMO 343 should focus on other mediums, stuff like Hunt the Truth
and the Official Spartan Field Manual
are much better additions to the canon. Prose just isn't a good fit.
I haven’t read a couple of the new ones from 2018 onward just yet, but I love the rest I’ve read. Picking a favorite for me means I’m only choosing which shiny object is shinier, which is to say they’re all pretty special in their own way. For instance, the Forerunner trilogy would not be my first choice to read again, but that’s purely because I prefer some stories over others and not because I can find fault with the trilogy itself.
The games have their issues, especially H5, but I think it’s safe to say the books have been consistently excellent thanks to the talented authors and they have only done good things for the Halo universe. Best of all, no microtransactions and no such thing as a “book as a service” ;-)
Sadly even this is no longer true. If your ebook comes with DRM you don't actually own it: