I can't really say there's a single favorite. I find pretty much every category has something that's appealing.

Old favorites I keep coming back to are the Fuzzy Books by H. Beam Piper, the first one is public domain. It's an easy, fun read. Shouldn't take longer to read than a few hours:

https://www.gutenberg.org/ebooks/18137

Other science fiction worth reading are the Matador books by Steve Perry, the Deathworld and Stainless Steel Rat books by Harry Harrison.

Fantasy? Obvious choice being the Hobbit and the Lord of the Rings, but a less obvious choice are the 10+ books of the Belgariad and Malloreon by David and Leigh Eddings, the Myth books by Robert Aspirin, Theives World anthologies edited by Aspirin, but are short stories written by all sorts of people. The Fafhrd and the Grey Mouser books by Friz Lieber.

If you're looking for more modern stuff, it's hard to beat the steampunk Clockwork Century books by Cherie Priest, or the horror/magic books of Night Watch by Sergei Lukyanenko.

Speaking of horror, Lovecraft is a must read, tons of good compilations of his stuff. Cthulhu mythos aside, my favotite stories of his are Cool Air, the Statement of Randolph Carter, and the Cats of Ulthar. Stephen King? Obviously the Stand, and the Shining, but the Dark Tower books are just marvelous. Well, the first FOUR are marvelous. Afer his van accident, the others aren't quite as good. Felt rushed. The first book, the Guninger, is an all time classic. The Clive Barker books are also good, but his BEST work, Imajica, is INCREDIBLY intimidating. If I remember correctly it's over 1,000 pages. That one took me 3 days to read. It's so thick that they split it in half to do the paper back edition.

Romance books? Hard to beat Outlander by Diana Gabaldon, originally written as Doctor Who fan fiction of all things! LOL. Shadow of the Wind by Carlos Ruiz Zafon is amazing, the language and turns of phrases used even in translation are stunning. The Griffin and Sabine books by Nick Bantock are fascinating, more of an art project than a straight narrative. You're encouraged to uncover the relationship between these two people through a series of letters and postcards, all faithfully produced through the books including envelopes.

For straight, non-genre fiction, you can't beat Umberto Eco. Name of the Rose, Foucalt's Pendulum, his collection of essays "How to Travel With a Salmon" is just wonderful.

For non fiction, I fell down a rabbit hole of travelogues for a while, Into the Heart of Borneo and In Trouble Again by Redmond O'Hanlon are fantastic, light and funny reads. His third book, No Mercy, about his trip to the Congo is not. It's really, really not. Very hard to get through, they were all almost killed. The Tim Cahill books are all worth a read, Road Fever being a particular favotite. Also the Michael Palin books, Around the World in 80 Days and Pole to Pole.

For religious theory I highly suggest Rescuing the Bible From Fundamentalism and Liberating the Gospels by Archbishop John Shelby Spong, the Screwtape Letters by C.S. Lewis (there's a fantastic audiobook version read by John Cleese of all people), and Issac Asmov's Guide to the Bible.

But that's just the highlights out of my library. ;)

I set up a little free library, here's the first batch of books I chose to give away:

The comic book room is a different conversation. LOL.