I really need to read some of the books from the last 5 years or so, I've heard they're really good. Fall of Reach was always my favorite, and was an important entryway into sci-fi for me (my mom would not let me read/watch sci-fi growing up, so I had to borrow a friend's copy back in high school)
I thoroughly enjoyed the mix of space opera/space fantasy that the Forerunner trilogy brought, and while they are controversial, I quite enjoyed some of the plots in the Kilo-Five Trilogy. I think sci-fi is at it's best when grappling with political and ethical issues, even if the analysis in Kilo-Five is a bit basic.
Honestly, I think some of Halo's best stories are in their short story collections. Loved most of the ones in Halo Mythos, and the stories Haruspis has made since joining 343 are super engaging.
Simone from Dispatches from Elsewhere was pretty pivotal for me when I first came out