I would have to say the Voyager Season 8 books have been great; right from "Homecoming" up to "Eternal Tide"
Worst, I would say that after a great start, the DS9 Season 8+ books got dealt some really bad stinkers with the Typhon Pact books. The first mini-series was a real clunker.
But ultimately the worst book would have to be "Fallen Gods". Some say that "Before Dishonor" deserves the honor of Worst, but at least it's more readable than "Fallen Gods". I'm still not sure how FG got past the editors with its long passages of useless description and a subplot about the Andorians that made no sense.