The Worf Academy books get all the attention, but I'd say my favorites were the two Picard books by Brad and Barbara Strickland, Starfall and Nova Command. Starfall is particularly potent, since it gets heavily into the tense family drama of Maurice and Robert Picard objecting to Jean-Luc's desire to join Starfleet, so it's a lot more dramatic than most of the Academy books. The Stricklands also did a Spock-focused one, Crisis on Vulcan, that's fairly good, though it's about the experiences that lead Spock to decide to apply to the Academy and ends just before Spock tells Sarek, so it skips over the kind of father-son conflict that Starfall embraces.
I found Diane Carey's Cadet Kirk kind of disappointing, since she contrives to put Cadet Kirk together with Ensigns Spock and McCoy on a shuttle mission, with no other characters beyond the villains they face. It's okay as a portrayal of their first meeting, but it feels like a missed opportunity, since most of the Academy books pair the familiar characters with new characters in new settings, allowing for different relationships and situations than we're used to seeing. Carey's book is just Kirk, Spock, and McCoy on a space mission, so it hardly feels different from an ordinary TOS story.
