Too many to choose from, but my favorites are non-fiction books mostly Science.
If I had to pick one it would probably be Death by Black Hole: And Other Cosmic Quandaries; by Dr. Neil DeGrasse Tyson.

Sci-fi/Fantasy
All time favorite: Song of Ice and Fire, George R. R.Martin
Least favorite: Revenge of the Witch, Joseph Delaney.

John Steinbeck is my favorite author and my favorite book is “Grapes of Wrath”.

My least favorite author would be Moses

My favorite is Bill Bryson, and I especially enjoyed his A short History of Nearly Everything.

I'd go Sci-fi with John Wyndham and Frank Herbert as front runners

I tend toward thriller / pseudo paranormal or sci fi stuff. My favorite authors for this today are Douglas Preston and Lincoln Child. I enjoy most of what they write together and separately. Douglas Adams is always a favorite too. My favorite book of all time might just be Wuthering Heights though. It is popularly portrayed as such a grand misguided romance but everyone is so horrible that I enjoy the darkness of the personalities and that most people don't appreciate what a dark story it really is.

For light, fun entertainment the Spencer series by Robert Parker. For life-changing learning, Man's Search for Meaning by Viktor Frankl.

Sorry can’t choose just one. I have been a reader since an early age. Currently read most mysteries/thrillers but have read science fiction, fantasy, young adult, romances ( not for a very long time), classics, and on and on. Can’t even name a book as today’s book may be replaced by tomorrow’s.

My favorites are science fiction and history.

Too many good ones to pick one favorite book. But for me, easily the worst book I read was "Seize The Day" by Saul Bellow.

I like mystery/thrillers and horror. I've been reading some Dystopian novels lately. I am chalking that up to the dystopian reality we are living in with Trump right now. I absolutely love John Connolly. His books are very dark. I like Stephen King and Dan Brown also. I love " Into the Forest" by Jean Hegelian. I have not read anything else by her. I have so many books that I love, that I can't pick just one.

Although I did just start reading a Stephen King book in Braille. Its going to be scary, I can feel it.

Bill Bryson, A short history of everything.

Military Sci-Fi.
Fav: The BOLO series by Keith Laumer.
Least Fav: Stephen King, everything post It.

I read science , science fiction, fantasy and whatever Terry Pratchett and Neil Gaiman call their special part of the fiction universe.

Sci-fi and fantasy, love Anne Rice!

My favorite genre is science fiction, but it needs to be creative and challenge me to consider things I'd never thought about before (e.g., moral dilemmas or some aspect of human nature or the nature of reality). I enjoy fantasy to a lesser degree. Thrillers and horror have their place, as does a good mystery. I generally prefer fiction over nonfiction.

My favorite book is probably The Fifth Head of Cerberus, which is actually three novellas each with its own style. My favorite author is probably Isaac Asimov, though there are so many greats. Mark Twain and Oscar Wilde rate very high.

The worst mainstream book I read was by the Dalai Lama, Ethics for the New Millennium. It was simplistic, excessively repetitive, and just boiled down to "do good things for others and you'll be happy." There's some truth in that, but I'm not sure a full book was needed to make that point.

Used to be coloring books. I don't read much... finishing Dracula, again. Frankestein next if I ever get to it. I am going to be reading for a while everything my daughter wrote on paper, everything she typed on her several computers, every piece of music she did.

I like different books for different reasons.

The book that first turned me on to reading, was actually assigned as reading when i was in high school. It ws "The Pearl" by John Steinbeck. Later I decided my favortie Steinbeck bok was actually "East of Eden", which I reread every once in a while.

Another book I like to reread is "Shogun" by James Clavell. However, I think Clavell's book "King Rat" was really good too.

For just good mindless entertainment, I like the "Disc World" book series by Terry Pratchett.

I also enjoy "The Butcher's Boy" series and Jane Whitefield series by Thomas Perry

I think Stephen King writes very well, but I p[refer his books that have no super natural aspects tothem, like "Cuho" of Delores Calyborn"

There are just way too nany good bolks to list them all.

"crime and punishment" is pretty good, but I'm going to say "the idiot"

Genre physics and mostly biographies. Too many favorites. "Broken Genius, the rise and fall of Wililam Shockley" by Joel N Shurkin stand out to me. Pretty much everything we enjoy today ( technology wise ) we thank to this guy but he has been sent to the back burner condemned by history to be forgotten because he spend his last years working on controversial ideas. Granted, that wasn't pretty and yet, his accomplishments should have credit the caliber of other important contributors. Least favorite books, there are quite a few that turned out to be hoax or big fiasco with claims of new theories that turned out to be less than a joke. Either way, reading has been and will be the greatest thing ever. How about you?

How to choose just one? That's unfair!

Umberto Eco is certainly a favorite author as I have read most of his works.

Least favorite....well, I was told that if I can't say something nice about someone.....let's just say any of those authors who churn out the same formulaic story over and over and over. The ones that you can read in about an hour and still feel hungry afterwards.

Salinger, especially everything except Catcher in the Rye.

I read mostly science fiction and classics. Dostoyevsky is probably my all time favorite. My favorite book by him is The Idiot.
Not sure about a least favorite. If I don't enjoy a book I usually just put it down.

Mystery/thriller Angels & Demons. Been reading some English authors and there are wayyy too many English idioms I don't understand, makes reading difficult--if I didnt have a friend in London I'd be lost--not reading them anymore.

I, honestly, can't pick a favorite genre. Taught English for 20 years with multiple readings and writings. My favorite author was and always will be Vonnegut because he is responsible for helping open my mind and think since school. I remember a teacher asking me, when he saw me carrying Crime and Punishment, are reading that or just carrying it for a reaction. I said, truthfully, both. Favorite book would be Slaughter House Five. Although, best book could be a multitude of possibilities, like Things Fall Apart by Achbe. Worst author or book? Let's just lump together anything by someone like Danielle Steele.
I would like to add this: there are some books that proved outstanding for society and need to be discussed, at least in the historical context, but are not very good reads today, my opinion would include Uncle Tom's Cabin, The Jungle, and most novels by Cooper, due to his Influences on the hatred of Native Americans. Cooper did, however , create the classic adventure tale.