I was rather surprised to see that Diane Carey had not cracked anyone's top five thus far. Admittedly, I have yet to read Ship of the Line which seems to have been universally panned on these boards, but I have very fond memories of reading Carey's books as an elementary/middle school student in the mid- to late-nineties. In particular, I would single out First Frontier and Best Destiny as among my all-time favourite Trek novels.
Rounding out my top five, in no particular order:
Judith and Garfield Reeves-Stevens (for Federation and the early Shatnerverse)
Peter David (most of all for Q-Squared)
Greg Cox (Q Continuum)
S.D. Perry (her DSN Relaunch novels)