I've read all of them. They're all worth reading, but, in my opinion, and I've pontificated about it enough in other threads so I'll be succinct. In my mind, the best 3, and the ones most worth reading are the ones by Eric Nylund, Fall of Reach, First Strike, and Ghosts of Oynx. The Flood by Dietz is one I consider to be a bit unfairly maligned. The guy was tasked with simply writing the novelization of the first game. SO, of course it can be a bit dry and repetitive at times, but, so is the game. Dietz did give us a good look at what else was going on the Ring with the rest of the UNSC personnel, we get our first real introduction to ODSTs, and I think Dietz was second only to Nylund in having best handle on the Master Chief’s personality.
For me though, Nylund is the Halo Expanded universe. He was where it started, he had the best stories, the best characterizations, and style best suited for the Halo universe.