Cradle! Or better, the cradle series. It's a sort of adventure story in a fantasy world.

The Locked Tomb series is refreshing. It’s weird, it’s fun, it’s dark, and it’s trash, but it’s trash that the author is having fun with.

Discworld is also just amazing

If you're into short stories the Illustrated Man by Ray Bradbury is a good one.

And while I didn't read much Issac Asimov myself my wife, who loves reading but dislikes sci-fi, read one of his books (Foundation) in a day and said he's an excellent writer.

I would recommend checking out audio books as a medium for reading. It allows you to increase the speed to whatever works for you, so 2x for me, and listen to a lot more in a day. It also frees you to listen at any times you have nothing cognitive happening, so dishes, washing, cleaning, etc.

As for single day books, the first book of the Bobiverse series by Dennis E Taylor. I loved the whole series including the recently released 5th book and the first is only 9.5 hours at normal speed, so about 4.75 at double speed.

Also All Systems Red is the first book in the Murderbot series by Martha Wells. The perspective of a SecUnit, a type of sentient cyborg, which has hacked its own programming and removed its limiters so it can act freely. This means no guard rails, no rules, no limits, which results in lots of TV shows being watched and avoiding humans. It is snarky, fun, and interesting. It comes in at 3.5 hours normal time, so 1.75 at double speed.

Recommend high quality short stories. Edgar Allen Poe has a collection that is some of the most thrilling, mysterious and fun, imaginative, adventurous, grotesque and other depending on the story. https://www.amazon.com/Edgar-Allan-Poe-Complete-Collection/dp/1453643141

Robert Louis Stevenson was also a fantastic writer of short stories.https://www.amazon.com/Complete-Short-Stories-Robert-Stevenson/dp/030680882X

I like short stories sometimes as I can't commit to a larger read.

The Lady Astronaut series by Mary Robinette Kowal. The first book is called The Calculating Stars. Basically, an alternate history where (spoiler for the opening chapter)

spoiler

a meteor wipes out the east coast and kick-starts climate change, causing the Space Age to start 10 years early.

It follows a Jewish computer (a woman who literally runs calculations for NASA, as seen in Hidden Figures) who wants to become an astronaut, and her struggles with the racism and misogyny of the 1950s.

I reccomend hopping on !chat@literature.cafe

I second someone else suggestion: the murderbot diaries. It's great.
Most of the books people here are recommending are fairly lengthy, but you can get through the first murderbot book in a dedicated evening.

Any early Alistair MacLean...

Guns of Navaronne

Where Eagles Dare

When Eight Bells Toll

Night Without End

Puppet on a String

Louis Lamour's westerns are complete popcorn and fun to read

C. S. Forester's Horatio Hornblower books

Others may have mentioned it (happy to see Terry Pratchett getting a lot of love), but would definitely recommend anything by Vonnegut! Love his writing style and his approaches to humor and world building. Slaughterhouse Five is a great one, as is Sirens of Titan.

Also, not certain how well they hold up, but I really enjoyed the Redwall series back in the day! I was much younger at the time, though.

The Handmaid's Tale

We

Nightfall

The Terminal Experiment

Not sure if I'd say they hit hard, but for readability it's hard to beat Rex Stout's Nero Wolfe books. Some of the best murder mysteries I've ever read, so much fun.

The Broken Earth series, Enders game series (the first 5 books about Ender), American Gods, An Absolutely Remarkable Thing and the follow up A Beautifully Foolish Endeavor, The Kingkiller Chronicle (we've been waiting 10+ yrs for the final book 3, some folks are pretty irked atp, but it will be ok). If you want YA beach reading, anything by Seanan McGuire / Mira Grant for easy fun books about fairies, cryptids, and zombies.

The Heart Shaped Box

NOS4A2

Between Two Fires

The Troop

The Princess Bride

Edit: Just realized you're looking for something to finish in a day, my bad. Have your read any Sherlock Holmes? They're entertaining and you can get through quite a few stories in a day.

I have just the book for you!... Ah, finish in a day, nevermind.

Completely ignoring your "short enough to finish in a day" instruction, try out Worm

I'm waist deep in The Dresden Files right now (just started Turn Coat, book 11 of like 20 and counting) and it very quickly became one of my favorite series I've ever read. Jim Butcher has woven a web of a story where every little detail is a foreshadow that often won't pay off until two books later, it's incredible.

Prior to this I read The Expanse and that one also comes highly recommended. It's one of the most believable space operas I've ever read. I also hear the TV show is good, no idea, never watched it.

When I was young, I read Diane Duane's The Young Wizards series, and I remember I loved it. Also Artemis Fowl, Sherlock Holmes, and The Inheritance series (C. Paolini). As an adult, I've read the LotR series which I highly recommend. Also, The Expanse series, 1984, Chronicles of Narnia.

Short enough to finish in a day...hmm that's tough. Maybe Screwtape Letters by C S Lewis? The Martian. Lots of short stories out there by Isaac Asimov!

"Short enough to finish in a day" seems pretty tough for me, but maybe I read slowly.

Short story books are good for casual reading in short sessions. Robot Dreams by Asimov, or Welcome to the Monkey House by Vonnegut. I used to carry each of those around and read a short story while waiting at a restaurant or at the DMV or whatever.

I really liked Altered Carbon. Approachable sci fi with drugs, violence, sex, politics, and of course high tech ideas like flying cars, AI hotels, digital consciousness.

Best? Hard to say. But favorite?

Galactic Pot-Healer by Philip K. Dick. It's quite short, like many of his books, and you could absolutely knock it out in a day.

Not sure about the length, but Vernor Vinge's Rainbows End is one of my favorite works of speculative fiction that really aged well so far.