Some might say that 'The Eugenics Wars: The Rise and Fall of Khan Noonien Singh, Books One and Two' is the worst. It tried to expand on Khan's backstory but ended up being overly long - winded. There were so many parts that dragged on and could have been told in a more concise way. Also, the character development wasn't as strong as it could have been for such an important character in the Star Trek universe.
I think 'Vulcan's Forge' is one of the worst. It had some really cheesy dialogue that didn't seem to fit the Star Trek style. The plot also had some holes in it. For example, some of the character's actions didn't seem to have proper motivation, which made the whole story feel a bit disjointed.
In my opinion, 'Dyson Sphere' is also a contender for the worst Star Trek novels. The science in it was a bit iffy. It didn't really hold up to what we know about the Star Trek universe's science. And the story was just not that interesting. It lacked the excitement and the thought - provoking elements that good Star Trek novels usually have.
Well, opinions on the 'worst' can vary greatly among fans. One that often gets criticized is 'The Final Reflection' by John M. Ford. Some fans didn't like its complex and convoluted plot that deviated too much from the typical Star Trek feel. It introduced a lot of new concepts and species in a rather clumsy way, making it hard for some to follow.
One that often gets the title of the worst is 'The Courtship of Princess Leia'. It has a plot that seems a bit forced. The way it tries to explore the relationship between the main characters feels unnatural. And it doesn't really enhance the overall Star Wars story but rather feels like a side - note that doesn't quite fit in.
I think 'Vulcan's Glory' could be considered one of the worst. The story just doesn't seem to flow well and the characters don't have the same charm as they do in the TV shows or better - written novels. It lacks the sense of wonder and discovery that is typical of Star Trek.
In my opinion, 'The Jedi Academy Trilogy' had its problems. While it had some interesting concepts, the execution was a bit off. There were some plot holes and the character development wasn't as strong as it could have been. Some of the new characters introduced didn't really gel with the existing Star Wars cast, and the overall story flow felt a bit disjointed at times.
I think 'The New Jedi Order: Vector Prime' was not great. It was the start of the Yuuzhan Vong invasion arc, and while it had potential, it just didn't quite hit the mark. The new villains were hard to understand and not as engaging as the classic Sith or Imperial foes.
Well, it's really subjective. But some fans might consider 'Spock's World' by Diane Duane as one of the worst. The plot can be a bit convoluted and the characterizations not as strong as in other Trek novels. It kind of strays too far from what makes Star Trek great in terms of its core values and the way the characters interact.
One of the worst could be 'The Crystal Star'. It had a convoluted plot that didn't seem to fit well with the overall Star Wars feel. The characters' actions and development were rather strange, and it was hard to get into the story.
The 'New Earth' series had its detractors. The plotlines in some of the books in this series were seen as rather convoluted. For example, some of the characterizations didn't seem to stay true to the original Star Trek characters. It was like they were trying to do too much with the new setting and lost sight of what made Star Trek great in the first place.
Good Star Trek novels are those that can make you feel like you're part of the crew. 'The Entropy Effect' by Vonda N. McIntyre does this well. It has a great mystery and the characters are well - rounded. In contrast, 'Prime Directive' by Judith and Garfield Reeves - Stevens is often considered bad. It takes the concept of the Prime Directive and twists it in a way that makes it seem more like a plot device than a fundamental part of the Star Trek philosophy.