just to make the compliment to the other thread.
what story would you count as least realistic (not most fantastic… just most “gee thats really dumb, no real world would ever be like that”)
I think the whole Foundation Series by Assimov, while great reads, are kinda dumb. Especially the Hari Seldon psychohistory junk.
The Number Of The Beast
I am prepared to be villified, however you spell it…
Shannara, or however the hell ya spell it. You know, that Brooks’ place. I have never found a book so unreadable, a world so poorly realized. I’ve tried three times to read the first book. Painfully unfinishable. My eyes ache thinking about it.
Jack L. Chalker’s The Well of Souls. Don’t get me wrong, I loved the world building and the story of Nathan Brazil, but credible?
Anything with faster-than-light travel is not credible, no matter how they explain it away. So knowledge of science tends to thwart the credibility of sci-fi.
NoClueBoy, I’d say the Heinlein future sci-fi books are far from credible, but because of character behaviour, not necessarily the specific sci-fi elements (except, again, the FTL travel.)
Are we talking about worlds that ignore the laws of physics, or just those that are not self-consistent? Larry Niven’s Known Space has lots of things that defy the laws of physics, like interstellar spaceships and stasis fields. But it’s very consistent and makes for a very entertaining setting, in the same way Discworld does.
On the other hand, Star Trek TNG has a very inconsistent setting because of all the one-episode gadgets. There are many cases where you wanted to shout “just use that thing from <another episode>!”
I never could suspend my belief enough to enjoy Pern. My daughters love the stories, though, and cannot get more than a page or two into Niven or Clement.
YMMV
The integral trees.
Jack Williamson’s early scifi novels hurt me deeply to read, particularly the first few installments of the Legion of Space.
Are there any improper trees? 
Well, it was the indefinite trees that I found truly unbelievable…
I’ll probably tick off a few people here with this one (it’s not intentional), but Piers Anthony’s Xanth always struck me as “fantasy-lite”… the world-building done there is as minimal as the characterization, and the world itself seems paper-thin, and inconsistent from book to book.
Xanth is my pick for least-credible fantasy world.
Flatland.
I haven’t read enough science fiction books to give a well-qualified answer, but let’s face it, the entire idea of Star Trek is pretty unrealistic. The faster-than-light travel bit aside, the entire idea of jetting through space, meeting hundreds of cool aliens, all who happen to look like us, is just a big fantasy. Some have argued that it’s the ultimate American fantasy.
The most probable truth is far more mundane: That other civilizations, if out there, are so few and far between that we’d never meet them or even know of their existence. Doesn’t make for good TV, though… 
Ring World.
Yeech.
I don’t know if this qualifies, but the world of Left Behind.
I mean, it’s not as though I have the greatest faith in the intelligence of my fellow humans, but if suddenly all the devout Christians disappeared, leaving their clothes, a new leader rose to power, etc., I’d like to think that most Americans (and Europeans, and any other culture that’s had decent exposure to the book of Revelations) would figure it out. Especially since the series seems to like to highlight scientific atheists. 'Cause I’m pretty sure that people disappearing and the like would qualify as proof.
Barring that, though, I don’t know…
Or the question that I have asked many people before along this topic:
Why does your fantasy have to be so real?
This actually bugs me. Back in the days when I played AD&D stuff like this came up ALL the time.
“No way a Vorpal Sword could do 400+ hit points damage! That kind of energy would melt the blade! DUH!”
Oh, so spell-casting elves and enchanting fairies are OK, but once you get to molecular structures and energy resistence, all bets are off? Puh-leeze… :rolleyes:
You are taking a leap of imagination, why should you choose to limit it? An old professor of mine told me about the time that he talked to his shrink about a dream he had:
“Doc, Last night I dreamed I won a million dollars.”
“A million? Why didn’t you dream that you won $100 million?”
The only related thing to the OP that sometimes pisses me off is the lumping effect. Everyone (-thing?) from another planet gets grouped for easy digestion. All Alpha Centaurians are X,Y and Z. All Googleplexians are _____ .
That is the sort of thing that doesn’t fit with me. That is like saying all Earthlings think like Americans. Ummm…no.
This is why I like [author plug] CJ Cherryh [/author plug]. Her worlds contain beings that DON"T THINK LIKE YOU OR ME. She takes Heinleins ‘Grok’ to higher levels…You CAN’T understand - their ALIEN, they don’t think like us!
Having a universe where the primary mode of transport is based on computer induced orgasms of microbes suspended in stasis fields surrounded by Playdoh doesn’t bother me. Having a world where every being thinks/reacts exactly like a white, college-educated, middle-class author from the Midwest would, does.
-Tcat
Xanth lost all credibility for me when it took the “stork” and “panties” stuff from being a cute double-entendre of adult misinformation told to children to being a real, tangible part of its “world.”
It doesn’t have to be “real” for me, Tomcat, but it does have to have depth and be consistent with itself. Xanth fails on both counts… it lacks depth and consistency, and thus totally fails to engage me as a fantasy world.
I can accept the elves in Tolkien or the magic in Melanie Rawn’s books, because they spend time giving the characters a rich, consistent world to interact with.
As far as I’m concerned, it’s the author’s fault if they don’t spend the time/energy buidling their world to a sufficient depth… the only “limits” placed on the imagination are the ones that they themselves have placed there.