The Fearful Summons is not only my least favourite
Star Trek book of all time, it's one of the worst books I've ever read, period. No one's in character (especially Spock, who's inexplicably becomes an
actor), the plot meanders all over the place, and the editing is atrocious - one character changes rank three times in two pages. I seriously thought about circling all the proofreading mistakes in red pen and mailing the book to Pocket Books with a note asking if they needed a proofreader.
Other than that, least favourites are:
The Prometheus Design
Triangle
The Fate of the Phoenix (Are we sensing a pattern?

Although actually, I quite liked
The Price of the Phoenix, oddly enough...)
Mudd's Angels
Spock, Messiah!
Probe (and yes, I know the back story of this book)
Enterprise: The First Adventure
Ghost Ship
The Devil's Heart
Ship of the Line
Masks
The Captains' Honor
There are a lot of
Trek books I didn't necessarily care for, but these are really the only ones I can think of where I had such a strong negative reaction that I will never read them again.