I find it immensely funny that the novels were numbered back when they had zero continuity, and were jumping around in the timeline with each book, and now that they're following a progression, there's no numbering at all, and you're just supposed to know.
Anyways, on topic,
TNG # 5: Strike Zone - Peter David's first Trek novel, and, while not his best, still better than most everyone else can do. I'm a shameless David fanboy.
TNG # 7: Masks - While it didn't quite live up to its premise, its got an interesting enough premise that I was willing to forgive it not quite following through as well.
TNG # 18: Q-in-Law - Arguably Peter David's best Trekbook. Pairing Q and Lwaxana was sheer genius, and as an added bonus, we get a Wesley subplot that's actually entertaining, and manages to make the character sympathetic!
DS9 # 11: Devil in the Sky - Hortas - wait for it - IN SPAAAAAAAAACE!!! Need I say more?
Really, a lot of the best stuff from the numbered period was actually the unnumbered books - the "giant novels" and hardcovers. Vendetta, Imzadi, Reunion, Dark Mirror, Q-Squared, etc.