I've read a few Deep Space 9 novels and found them fairly enjoyable, but generally not really as good as The Next Generation ones. Having said that, this is purely my own personal tastes and not a criticism of the series, which I have just started watching again on DVD from episode 1. Sorry I can't be more helpful.
I can recommend the Double Helix Series
(Infection, Vectors, Red Sector, Quarantine, Double or Nothing, The First Virtue
) and the Invasion Series
(First Strike, The Soldiers of Fear, Time's Enemy, The Final Fury
) however, since they have crossovers between all the different series (except Enterprise, which I never really counted as being true
Star Trek). Both have different types of theme, great storyline and pace, as well as benefiting from the input of multiple authors (a different one for each book).
The difference between fiction and reality? Fiction has to make sense.