I have always really liked Mirror Universe: Sorrows of Empire. It’s by far the book I’ve read most but I do have a large in house library of Star Trek books because people sell bundles on eBay for reasonably cheap.
THE NEW FRONTIER collection of books 1-4 if that's not cheating. After that, I have THE DEPARTMENT OF TEMPORAL INVESTIGATIONS: WATCHING THE CLOCK by Christopher Bennett.
Spock's world by Diane Duane and And Her series of Romulan books.Corona by Greg Bear. Elusive Salvation by Dayton Ward. Child of 2 worlds by Greg cox .Savage trade by Tony Daniel .Ds9 Crimson shadow by Una McCormack. Picrd Second self by Una McCormack.
Sarek by Ann C. Crispin The Neverending Sacrifice by Una McCormack The Terok Nor novels by S.D. Perry and James Swallow Fallen Heroes by Dafydd ab Hugh Just to name a few.........
Oh, gods... There's no way I can pick a favourite, but if I could it would probably be one of my most read - either Diane Carey's Final Frontier, Judith & Garfield Reeves-Stevens' Prime Directive, Margaret Wander Bonano's Dwellers in the Crucible, TLE: Well of Souls (Ilsa J. Bick), or The Lives of Dax anthology edited by Marco Palmieri. But then the Vanguard series is absolutely awesome, and I love The Never Ending Sacrifice (Una McCormack), TLE: The Art of the Impossible (Keith R. A. DeCandido), Avatar I & II and Unity (S. D. Perry), and Watching the Clock (Christopher L. Bennett).
Not only do I love the worldbuilding with the Romulans and the story of Spock and Saavik...it just has two of my favorite characters ever (who happen to both be Romulan): Charvanek, from "The Enterprise Incident", and one of the coolest Romulans ever, Ruanek. I hope they survived the destruction of Romulus!
I really enjoyed the Saavik and Spock relationship so I was a bit sad so much of the book was them dying or being super-horny. But the Romulan intrigue was exceptionally well-done and I loved seeing some of Captain Garrett's adventures.
Mission to Horatius. Just kidding: actually, it's Spock: Messiah. Still pure bovine scat; the only reason I had S:M (which kind of says it all) hardbound was because it was only the second ST novel written for an adult audience (and it put the "adult" in "adult ST fiction"). Would you believe, The Price/Fate of the Phoenix? No, of course you wouldn't. They make the intentionally-confusing-for-comic-effect scenes near the end of How Much for Just the Planet seem as simple as direct current, and I don't think they were intended to be confusing, and certainly not for comic effect. In all seriousness, Spock's World. I've probably read it more than any other single ST novel. Although I've read Diane Carey's Piper books and her Geordie Kirk books quite a few times as well, along with Jean Lorrah's The Vulcan Acadmey Murders. My love of Spock's World would be easy enough to guess from my reaction to what "Reanock" said.
Over forty years old, and it’s still The Entropy Effect for me. I read it for the first time a month or two before TWOK came out, and all these years later, they are my favorite Star Trek novel and movie. It was a golden age.
The two books I have re-read the most times (I figure that's as good a barometer as any) are A Stitch in Time and The Neverending Sacrifice.
Vulcan's forge By Josepha Sherman and Susan Schwartz Also Strangers from the Sky by Margaret Wander Bonanno. The Tos era Vanguard series of books and Spinoff books The seeker series.